Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/718

700  The above scheme is a nail in a sure place ; and on it, for the present, we may hang all that we know, or are learning, of the anatomical structure of this class of Verte brates. That which relates to the Carinatoe must, how ever, be regarded merely as a list of Birds having a similar facial structure. For the general ornithologist it is very suggestive and helpful, and will save him from looking merely on outward appearances; for the study of structure and development is looking into the heart of the matter.

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To both the zoologist and the palaeontologist an ex planation of the skeleton will be of the greatest value, for the framework must of necessity be correlated to the nervous system, and also govern the development of the muscles. It will here form the first and the largest part of our work. And as all things in the skeleton are con formed to the modifications of the skull, and, moreover, as the skull is the most knotty problem to the morphologist, it will receive the attention due to its superior importance. Instead of describing the adult skull, and then showing how it develops, it would seem to be better to follow the stages of its growth, and thus see the meaning of the parts, and what metamorphic changes take place to give it its adult characters. Space will not permit any detail of the general embryology of the Bird, but the skull will be described from the time when the rudiments of the chondro-cranium are first fairly visible, that is, about the fifth day of incubation. As the Schizognathous type of skull, such as is seen in the Fowl, is the simplest variety found in the Carinatce, it will be the most convenient for comparison with that of the Ratitce and the Tiuamous (the Dromceognathous variety) below, and the Desmognathous, and other kinds seen in Birds above the GallinacecB in the zoological scale.

The Cranium of the Fowl—First Stage.—The chondro-cranium may be seen at the end of the fourth and the begin ning of the fifth day of incubation, although the cartilage has as yet but little consistence, its cells being imperfectly soldered together. The head of the skull at this stage still shows the &quot;visceral clefts;&quot; and it is bent upon itself by what is called the mesocephalic flexure. When the membranous roof of the skull and the brain are removed at this stage, the whole floor is not seen from above, the fore-part being bent under and looking backwards.

In the hinder half of the skull-floor, behind the eyeballs, we see a broad plate of tis sue (fig. 1, comp. fig. 4, i.v.) which is passing rapidly from the condition of stellate cells into proper hyaline cartilage. This plate is divided at the mid- line by a straight, somewhat beaded rod of soft indif ferent tissue, which does not chondrify ; it is rounded at its fore-end. This truly azygous part is the notochord (n.c.), or primary axis of the skeletal parts of the embryo; it lies directly beneath the neural axis, and is one of the parts earliest visible to the embryologist. The broad plate on each side is seen to be hollow, and to contain a pear-shaped diverticulum of the primordial ear- sac (d.), which is planted, as it were, in the very sub stance of the basal plate, at its middle. The outer granular covering of the ear-sac becomes cartilage, and so does the basal plate the &quot; investing mass &quot; of Rathke, the &quot; parachordal cartilage &quot; of Huxley. But the process by which these two separate morpho logical territories become converted into solid cheese-like cartilage is carried on blindly, as it were, and no distinc tion of parts is at present traceable ; evident differentiation of morphological territories is often late in the higher types of vertebrata. The tissue which surrounds the spinal chord where this part passes into the brain is still soft ; it will chondrify soon to form the occipital arch. The bulbous end of the notochord ends a little behind an oval membranous space or f ontanelle in the skull-floor the pituitary space (pt.s.). Near the end of the notochord, on each side, the cartilage suddenly narrows, for here we are in front of the impacted ear-sacs; at this part a bending of the narrow anterior end of the parachordal rounded notch (5) is formed, over which the trigeminal nerve passes. The anterior margin of the notch (after wards formed into the foramen ovale) is formed by the cartilage; its actual extremity looks forward and outward, towards the eyeball. At present the bands which are continued forward, surrounding the pituitary space, are superimposed upon, and indistinct from, the ends of the parachordal cartilages; in the next stage they will be seen more distinctly. These flat bands of dense granular tissue are the trabeculce cranii of Rathke (tr.), and they were supposed by him to be mere continuations of the para chordal bands, an error corrected some years ago by Pro fessor Huxley. These little rafters of the cranium bend gently round the oval pituitary space ; they then com- 