358 Beard, Heredity and the epicycle of the germ-cell. 



the nineteenth century, thought differently, and some pathologists still 

 cling- to their views, but these have no shadow of foundation in fact. 



The initial mode of growth and formation of the asexual generation 

 or larva in animals - - an organism never of a very high degree of 

 organisation is entirely comparable to that of the sporophyte. As in 

 simple cases of the latter, there is here one ,,apical cell", which never 

 itself forms part of the larva, but instead thereof gives off into the 

 latter a greater or less number of products, while retaining its own 

 unicellular or Protozoan character. Nor would the conditions be altered, 

 if there were several growing points, as generally met with among the 

 Hydrozoa 1 ). 



It may be objected, that whereas the early cleavage of Nereis, 

 Ascaris, etc., is spiral, in the Vertebrata, such as the skate, it is bi- 

 lateral. The objection would not, I think, be a valid one. The 

 meaning of such a bilateral cleavage in the early development as- 

 suming it to exist - - would simply be, that there were two spirals 

 instead of one, and, possibly, two primitive germ-cells. For various 

 reasons I regard the actual larva or phorozoon 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 

 or larva, so also in the tadpolelike 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. 



Returning 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 differentiated, 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 orphoro- 



3) It should be mentioned that de Vries and Weismann have already 

 noted the resemblance in mode of growth between the sporophyte and the 

 colonial Hydrozoa. Many of the latter also possess the indefinite unrestricted 

 power of growth, so characteristic of the sporophyte of the higher plants, As 

 a rule the asexual generations of the higher Meta/oa 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 abortion in the human subject, the embryo is got rid of prior to the 

 critical period, or at any rate before the asexual generation has here been sup- 

 pressed, the latter may go on growing indefinitely, if left in the uterus. I refer, 

 of course, to the unrestricted and pernicious growth of the chorion, when left 

 in the womb after an abortion. 



