Page:Popular Science Monthly Volume 64.djvu/111

Rh kinds of spermatozoa has lost its power of fertilizing the egg and in most cases has become degenerate.

In respect to the occurrence of the two kinds of spermatozoa, Beard brings together rather a heterogeneous collection of facts. It has been known for some time that in a few cases two kinds of spermatozoa are found. The oldest and now the most thoroughly studied example is that of the snail, Physa vivipera. In this animal there are hair-like spermatozoa that resemble the ordinary forms of spermatozoa, and also worm-like forms which are as numerous as the other kind. A remarkable fact has recently been discovered by Meves in regard to these spermatozoa. An unusual and probably degenerate process occurs in the formation of the worm-like spermatozoon, so that instead of containing the reduced number (seven) of chromosomes it contains but a single one. In another form, Pygæra, this second form of spermatozoon contains no chromatin material whatsoever, i. e., it is headless and presumably functionless as well.

In the long list of cases given by Beard in which two forms of spermatozoa have been described, there are several cases in which the two distinct forms appear to be always present and characteristic, as in the cases cited above; but he has also included some other cases in which giant spermatozoa occur, and some of these at least have been shown to be the result of a failure of the spermatocytes to divide. Until it can be shown that this failure to divide is usual and characteristic of one set of these spermatocyte cells the result may really have no bearing at all on Beard's contention.

Much more striking are the cases in which there is an accessory chromosome present in two of each of the four cells that develop from a single spermatogonial cell. The discoveries of McClung, Montgomery and Sutton in this connection indicate that there are two kinds of spermatozoa, and McClung has urged that this difference is connected with the determination of sex; but there is nothing more than the supposition that this may be so to go upon at present. In these cases, although the form of the spermatozoa is the same for the two kinds, there appears to be a difference in the amount of the chromatin material. It has not been shown that a difference of this kind would have any value in the determination of sex, and even if this were the case the results do not conform to the requirements of Beard's theory, as we shall see presently.

Beard calls attention to the fact that in nearly all the cases in which two kinds of spermatozoa have been described there is evidence of the degeneration of one of the two kinds. Prom this he draws the rather sweeping conclusion that throughout the animal kingdom one of the two forms of spermatozoa has become suppressed. He arrives at this conclusion in the face of an overwhelming body of evidence