72 CLEAVING OF THE YOLK 



From the preceding account of the three principal varieties of the pro- 

 cess which intervenes between the disappearance of the germinal vesicle 

 and the formation of the embryo, in Entozoa and many other invertebrate 

 animals, it will be apparent, as Kolliker remarks, that the more essential 

 part of the process is the development and multiplication of nucleated 

 embryonic cells, which become smaller as they increase in number, and at 

 length form the granular mass out of which the embryo is moulded. The 

 part played by the yolk appears to be a subordinate one: the peculiar 

 process of division and subdivision, which, in some cases, it undergoes 

 whether this affects its whole mass, or only a limited portion of it being 

 determined and regulated by the development and increase of the em- 

 bryonic cells. 



In Amphibia and Fishes. It remains to inquire how far the phenomena 

 observed in the ova of vertebrate animals agree with those which we have 

 just been considering: and, first of all, to apply the facts with which we 

 have become acquainted, to the explanation of the analogous process in 

 Amphibia and Fishes. 



That variety in the process which consists in the whole yolk undergoing 

 division and subdivision, was discovered long since, in the ova of frogs, 

 by Prevost and Dumas,* and afterwards described with great accuracy by 

 Von Baer.f Baer, however, seems to have entertained no suspicion that 

 the segments of the yolk are other than solid homogeneous masses of the 

 yolk-substance. Rusconi, J in criticizing Baer's account of the process, 

 remarked, that there were cavities in each of the eight masses into which 

 the dark half of the yolk divided, and was probably led to make this 

 remark by seeing the transparent embryonic cells. But Bergmann first 

 announced that each of the masses of the yolk contained a transparent 

 body, which he supposed to be a solid nucleus. 



From these observations, though imperfect, the most natural inference 

 is, that the process described by Bagge and Kolliker, as taking place in 

 the ova of Strongylus auricularis, and Ascaris nigrovenosa, and acuminata, 

 takes place also in the frog, and that the division and subdivision of the 

 yolk is dependent, in the latter animal, as in those Entozoa, on the develop- 

 ment and multiplication of embryonic cells. Kolliker|l indeed states, that 

 he has seen nuclei in the vesicles which occupy the centre of the segments 

 of the yolk in the frog's ovum. The earlier statements of Reichert,1[ too, 

 respecting the structure of the yolk at the end of the process of cleaving 

 are also reconcilable with that view. The smaller corpuscules, he describes, 

 may be the last generation of the embryonic cells, still surrounded by 



* Muller's Physiology, p. 1508. t Ibid. p. 1509, and fig. 168. 



J Muller's Archiv. 1836, p, 218. 



Muller's Physiology, p. 1509 note ; and Mliller's Archiv. 1841, p. 89. 



|| Entwickelungs-geschichte der Cephalopodcn, p. 121. 



H Muller's Physiology, p. 1512. 



