406 Beard, Heredity and the epicycle of the gerin-cells. 



and the critical period is reached, the embryo contains only secondary 

 germ-cells, incapable of independent development. 



It may be of interest to record the further fact, that in the skate 

 this formation of secondary germ-cells precedes the announcement of 

 the sex of the embryo, and is possibly causally related to it. Aswe 

 have already seen, the future sex is betrayed by the nature of the 

 egg itself. It is announced by differentiation of ovary or testis. 



So far as the germ-cells themselves are concerned the union of 

 the paternal and maternal nuclear portions is one of no long duration ; 

 for a commencement of the undoing of it is made at the formation of 

 the secondary germ-cells. That is to say, the union brought about by 

 the conjugation only persists until the primary germ-cells cease to 

 be such, and divide to form secondary ones. Moreover, the tendency 

 of research goes to demonstrate a certain looseness in this union. As 

 Kiickert and Hacker independently showed a few years ago, the 

 paternal and maternal chromosomes remain distinct during the cleavage 

 of Cyclops, indeed, in such a manner as to suggest a duplex-nucleus 

 in each of the cells along the germinal track (in Weismanu's sense). 

 The like observation was subsequently made by the former in the 

 cleavage-cells of Torpedo, without, however, suggesting any connection 

 with germ- cells. The same duplication was recently noted by the 

 writer in cleavage-cells of Raja, as well as in the primary germ-cells 

 here, and the two were brought into connection. The interesting point 

 about the matter appears to the writer to be, that if the reduction of 

 chromosomes at the determination of sex for the following generation 

 be ever a mere undoing of the previous lax union, the resulting germ- 

 cells may reproduce or mimic more or less exactly, if not indeed ab- 

 solutely so, the corresponding germ-cells of a grandparent. After 

 such a reduction and new conjugation the reduced halves of the nuclei 

 have, of course, lost the previous loose union referred to above, in 

 order to acquire a new one of the like nature. Therefore, they cannot 

 so easily revert to a great-grandparent. It is, I take it, the looseness 

 of this union of chromosomes and the ease with which it may be un- 



Though there be no preformation, there is a predestination, and this is finally 

 brought to pass by an evolution or unfolding. In my own work the facts of 

 the development of the thymus, of the lateral sense organs, of the whole 

 gut, etc. are only explicable and intelligible on this view. Evidence of the like 

 kind is also afforded by the facts as to the developmental origin of identical 

 twins. The very instance, chosen by Caspar Friedrich Wolff, that of 

 the development of the alimentary canal, in reality demonstrates in the clearest 

 fashion, that its history is one of an evolution. The detailed facts concerning 

 this may be brought forward on an early occasion. 



With Weismann I must emphatically maintain epigenesis to be an im- 

 possibility in Metazoan development. 



