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Rh the embryo so as to include the mouth. But the independent and late development of the mouth is as marked a feature in the echino- derm larva as in the vertebrate embryo and there are no gill pouches in the echinoderm on which we could fall back to explain the phenom- enon. Further, in the adult echinoderm the whole of the ectoderm is underlain by a nervous plexus of which the central nervous system is only a specialized and intensified portion, and the same thing is true of the anterior region of Balanoglossus. It is therefore futile to look for exact correspondence between the central nervous systems of two stocks which diverged from one another at such a primitive level as did the Vertebrata and the Annelida. In fact the descendants of the trochophore stock (Annelida, Arthropoda and Mollusca) on the one hand and the original Vertebrata on the other seem to have adopted two different modes of life which led to concentrations of the nervous system in different parts of the body. The trocho- phore stock took to crawling on their ventral surfaces and their locomotor organs were developed in this region of the body and in connexion with them the motor ganglia which make up the ventral nerve-cord ; whereas the vertebrate stock took to swimming by lateral blows of their blade-like bodies and this led to the concen- tration of the central nervous system in the mid-dorsal line.

Enough has been said to give evidence for our belief that the most recent research has tended to reestablish the recapitulatory element as the fundamental factor in life-history, and if this be admitted the study of comparative embryology opens up a means of investigating the early history of life at a stage long before it left evidence of its existence in the stratified rocks; and, further, the acceptance of recapitulation involves a conception of the laws of heredity entirely distinct from and supplementary to that suggested by Gregor Mendel and his followers.

BIBLIOGRAPHY. F. R. Bailey and A. M. Miller, Textbook of Em- bryology (1909) ; F. M. Balfour, A Treatise of Comparative Em- bryology, vols. i.andii. (1881) ; A. Brachet, " L'CEuf et les Facteurs de 1'Ontogenese," Encyclopedic Scientifique, Paris (1916); J. C. Heisler, Textbook of Embryology for Medical Students (1907); H. Driesch, The Science and Philosophy of the Organism; Gifford lectures for 1907 and 1908; O. Hertwig, Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbelthiere, Jena, 18901920; J. W. Jenkinson, Vertebrate Embryology (2nd ed. 1921), Experimental Embryology (1909), Three Lectures on Experimental Embryology (1917); J. Graham Kerr, Textbook of Embryology, vol. ii., Vertebrata (exc.

Mammalia) (1919) ; Korschalt and Heider, Textbook of the Embryology of the Invertebrates, translated by E. Mark, Ph.D., and W. McM. Woodworth (1895); Lillie, The Development of the Chick; E. W. MacBride, Textbook of Embryology, vol.i., Invertebrata (1914); A. Milnes Marshall, Vertebrate Embryology (1893); T. H. Morgan, Regeneration (1901). (E. W. MAcB.)

ENCEPHALITIS LETHARGICA (from Gk. ifKipaXov, a portion of the brain, and Ai70a/ryos, forgetful), a specific infectious dis- ease of the nervous system, of which the most frequent, though by no means invariable, symptom is drowsiness or lethargy, often associated with paralysis of the muscles of the eye, pro- ducing diplopia or double vision.

In recent years this disease first appeared in epidemic form in Austria in the winter of 1016, and was described and named by C. von Economo in Vienna in 1917. It reappeared in the following winter, and was recorded in France in March 1918 by A. Netter. In England it was first recorded in April 1918, independently by Wilfred Harris and A. J. Hall. These earlier epidemics were all recognized by the combination of lethargy and diplopia. Subsequently the same seasonal incidence has prevailed, the number of cases diminishing in summer and increasing in winter and spring. In the United States it was reported in March 1919, the epidemic having spread from east to west.

The literature of medicine has been ransacked to ascertain whether or not previous records exist of the occurrence of the characteristic combination of symptoms. In 1890 a small epi- demic occurred in North Italy, and later in Austria and other countries, which may be accepted as Encephalitis Lethargica. At the time it was known as Noma. It attracted little attention. The evidence for sporadic cases since then is doubtful.

Previous records are all doubtful. Hippocrates, Sydenham and other less-known authorities have described conditions which may or may not have been Encephalitis Lethargica. Stahl in 1779 recorded more fully an outbreak characterized both by lethargy and diplopia, and with other very suggestive symptoms. Franck in 1837 divided Encephalitis into various types, including a lethargic form which he stated occurred frequently as a result of epidemics of influenza.

Whether or not the observed combinations of symptoms have ever occurred before, the question early arose whether the syndrome should be regarded as a clinical entity or as a special manifestation of some disease previously known in other forms. The three diseases with which its relations have been more particularly considered are botulism, influenza and acute polio- myelitis or Heine-Medin's disease. Botulism results from the con- sumption of infected meat or other food, usually ham, and is espec- ially characterized by paralysis of the eye muscles. The disease is due to infection with a specific bacterium the bacillus botu- linus. Since the most careful search has invariably failed to detect the presence of this bacillus in Encephalitis Lethargica botulism can be finally excluded. With regard to influenza, the appearance of Encephalitis Lethargica, including Noma as such, has approximately coincided with epidemics of influenza in 1889 and at the present time. On the other hand, there is considerable evidence against their identity. Influenza is highly contagious, while the clinical appearance of Encephalitis Le- thargica occurs in an irregular manner and has not attained any great magnitude as an epidemic. When Encephalitis Lethargica commenced in Austria influenza had not yet appeared. There are also histological differences, since in influenzal encephalitis there is marked oedema of the brain and an absence of the two special characteristics of Encephalitis Lethargica vascular congestion and infiltration of the perivascular lymph spaces. Pfeiffer's bacillus, the so-called influenza bacillus, is not found in Encephalitis Lethargica.

The relations of Encephalitis Lethargica to Heine-Medin's disease have led to much discussion. Unusual forms unquestion- ably occur in which the diagnosis is doubtful, both on clinical and pathological grounds, but, considering typical forms, there are striking differences between the two diseases. Heine-Medin's disease particularly attacks persons under 20 years of age, and tends to increase in frequency in summer. The onset of the general symptoms and of the paralysis is acute, the course is brief and the spinal cord is mainly affected. In Encephalitis Lethargica, on the other hand, persons of all ages are liable to attack, and the frequency is greatest in winter and spring. The onset is usually insidious, the course is lengthy, and the mid-brain is especially affected. Histologically there are also important differences. Microscopic haemorrhages are constant in Heine-Medin's disease, while perivascular infiltration is slight, the reverse being true of Encephalitis Lethargica.

Symptoms. Encephalitis Lethargica was primarily recognized by the occurrence of the combination of lethargy and double vision, the latter being due to paralysis of the muscle of the eye. While pathological drowsiness in varying degrees is probably present in 70 to 80 per cent of cases at some stage of their course, further ob- servation has revealed the occurrence of numerous other mani- festations. The disease is widespread through the nervous system, and the complexity of the structure of the nervous tissues, together with the high degree of specialization of the functions of its various parts, explains the protean nature of its symptoms. Numerous " types " have been described, but the value of these is slight, as a single patient in the course of a few days often exhibits the char- acteristics of many such types. The clinical manifestations are probably best classified as Walshe suggested, according to the broad scheme proposed for other nervous diseases many years ago by Hughlings Jackson. In the following description based on this system, " positive " symptoms denote exaltation of function, which may be due either to irritation of nervous tissue or to a loss of the control exercised normally by the higher centres of the brain, while " negative" symptoms denote depression or loss of function principally due to destruction of nervous tissue.

A. General Symptoms Due to Toxic Infection. These include weakness, headache, often occipital with some stiffness of the neck, shivering, vertigo, muscular pains and vomiting or other gas- tro-intestinal disturbances. The pulse may be rapid and eruptions, usually resembling measles, occasionally occur. The temperature is variable, and it has no characteristic course. It often rises after some days from 101 to 105 F. for a short period, but may be more prolonged, or pyrexia may be absent throughout.

B. Nervous Symptoms. These are general and focal, the latter being due to affection of highly specialized portions of the brain.

(I) General Nervous Symptoms. Positive symptoms are delirium, mania, restlessness and various degrees of excitement, while the more common negative manifestation is the characteristic lethargy, in all grades from simple apathy to complete coma. Innumerable degrees occur of these two extremes, or even combinations. The