IN INVERTEBRATE ANIMALS. 67 



masses, corresponding to the four cells which they enclose within them 

 (B). This process of division, by which each cell, and consequently each 

 mass of yolk-substance, is resolved into two others of half the size of the 

 original, is repeated until the yolk is converted into an oval mass com- 

 posed of globules, in the interior of each of which, the existence of a 

 nucleated cell can be distinctly discerned (C). Beyond this point, however, 

 the cells can no longer be recognised, though the process of division 

 continues, and the globular masses become smaller and smaller (D, E), 

 until, eventually, they begin to be moulded into the form of the young 

 worm, in the construction of which they all take part. 



Such is a general outline of the process as it occurs among these 

 varieties of Ascarides, and has been observed also among several other 

 invertebrate animals. There are certain questions, however, which present 

 themselves for solution in the further consideration of it. These are 



What is the mode of origin of the first cell seen within the yolk after 

 the disappearance of the germinal vesicle "? 



What is the mode of multiplication of this cell \ 



How is the division of the yolk produced 1 



Are the different segments of the yolk to be regarded as cells 1 



Neither Bagge nor Kolliker observed any facts which afford an answer 

 to the first question. For they were unable to perceive the first em- 

 bryonic cell within the yolk until it was completely formed. This much, 

 however, seems certain, that the cell is not developed from the germinal 

 spot or nucleus of the germinal vesicle. For in the perfectly transparent 

 ova of Ascaris dentata, Kolliker satisfied himself that the germinal spot 

 as well as the germinal vesicle invariably disappeared, and that a certain 

 period elapsed between their disappearance and the formation of the first 

 embryonic or germinal cell. 



With regard to the mode of multiplication of the cells of the yolk, 

 Bagge and Kolliker are at issue. Bagge, who appears not to have 

 observed that the cells possess nuclei, represents them as undergoing 

 division much in the manner of the fissiparous generation of polygastric 

 animalcules. But Kolliker has shewn that each cell is nucleated, and that 

 sometimes one large cell contains two small nuclei in place of one of 

 twice the size. And as he has observed in other Entozoa that the same 

 cells multiply by the development of two young cells from the two halves 

 of the divided nucleus within the parent cell, which then disappears, he 

 draws the fair inference that the same process occurs in these Ascarides 

 also. A similar opinion is entertained also by Dr. Sharpey.* 



It would seem that the division of the yolk is a consequence of the 



nucleated cell is clearly discernible. From Ascaris nigrovenosa. (After Kolliker.) D. and E. 

 are representations of ova from Ascaris acuminata, shewing subsequent steps in the process 

 of division described in the text. (After Bagge, op. cit.) 

 * Quain's Anatomy. Fifth edition, p. 1. 



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