J.vx. 1902.] BOTANICAL SOCIETY OF EDIXBUEGH l-tl 



It may be objected that whereas the early cleavage of 

 Nereis, Ascaris, etc., is spiral, in the Yertebrata, such as 

 the skate, it is bilateral. The objection would not, I 

 think, be a valid one. The meaning of such a bilateral 

 cleavage in the early development — assuming it to exist — 

 would simply be that there were two spirals instead of 

 one, and possibly two primitive gerra-cells. For various 

 reasons I regard the actual larva or phorozoou of the 

 skate as at the basis very like the tadpole larva of 

 Ascidians. Indeed, I would go further, and, following 

 the example of Roule with his classification of certain 

 Invertebrate groups, as " Trochozoa," by their asexual 

 generation of larva, so also in the tadpole- like larva of 

 the Ascidians I would see — not the Vertebrate relation of 

 many embryologists — but the like or even homologous 

 asexual generation of Ascidians, Amphioxus, and the true 

 Vertebrata. 



Keturniug to the diagram. Sooner or later upon the 

 larva the primitive germ-cell enters into activity. It may 

 divide before the larva or phorozoon is properly ditteren- 

 tiated, as nowadays is certainly the case in many instances, 

 or, theoretically, its divisions may happen at a later period. 

 These divisions, however, must precede the formation of 

 the embryo or sexual generation. 



In the skate the divisions of the primitive germ-cell, 

 which give birth to the primary germ-cells, take place 

 before the larva or phorozoon is fully differentiated, and, 

 of course, before there is any trace of the embryo. 



For reasons to be fully given in my memoir on the 

 germ-cells, the divison of U.K.Z. the primitive germ-cell, 

 is considered to go back to about the tenth cleavage pro- 

 ducts, and in the skate there are either eight or nine 

 divisions. 



unrestricted power of growth so characteristic of the sporophyte of 

 the higher plant?. As a rule the asexual generations of the higher 

 Metazoa do not exhibit this faculty. They rarely obtain a chance of 

 showing it, for it is their usual fate to undergo early suppression by 

 the sexual generation. When, as happens sometimes in cases of abor- 

 tion in the human subject, the embryo is got rid of prior to the critical 

 period, or, at anvrate, before the asexual generation has here been 

 suppressed, the latter mav go on growing indefinitely, if left in the 

 uterus. I refer, of course^ to the unrestricted and pernicious growth 

 of the chorion Avhen left in the womb after an abortion. 



