74 CLEAVING OF THE YOLK 



situated : the contents of each cell consisting of the granules and lamellae 

 of stearine, which previously constituted the yolk, These facts accord with 

 the observations of Kblliker, on the sepia and intestinal worms. But 

 other parts of Vogt's description cannot be reconciled with Kolliker's 

 views : for instance, the statements that the cleaving of the yolk is pro- 

 duced by folding in of the yolk membrane : that some of the segments have 

 no cell-nucleus, while others contain several: and that the formation 

 of the cells last described, is not coincident with the cleaving, but begins 

 after the cleaving-process has wholly ceased a space of time intervening 

 in which nothing like cells exist. Vogt believes, too, that the transparent 

 cells which are contained in the segments of the yolk, are multiple germinal 

 spots, which remain when the germinal vesicle, in consequence of impreg- 

 nation, disappears, and which exist in the yolk before the process of 

 cleaving commences (see page 63). Between Vogt's account of the 

 process in Coregonus, and Kolliker's description of the division and subdi- 

 vision of the yolk in Sepia and other Invertebrata, there is much apparent 

 discrepancy. Kblliker, however, thinks that they may be reconciled. As 

 there is, however, much that is indefinite in Vogt's description, further 

 consideration of it need not be entered into here. It may be remarked, 

 however, that Vogt is of opinion, that here also the germinal spots play an 

 important part in bringing about the changes in the yolk, which precede 

 the formation of the embryo. 



In Birds, nothing like a cleaving of the yolk has hitherto been ob- 

 served. It is supposed that the extent of the yolk implicated in the process 

 may be very limited. It may be, perhaps, that the process does not affect 

 the yolk itself, but consists solely in the development and multiplication of 

 embryonic cells, in the central cavity of the yolk, such as is described by 

 Kolliker in Ascaris dentata and Cucullanus elegans (see page 71, and fig. 8), 

 but without the simultaneous absorption of yolk substance, such as takes 

 place in those Entozoa.* 



In Mammalia. Lastly, the nature of the process which takes place in 

 the ova of Mammalia must be inquired into. 



The changes which the yolk of the rabbit's ovum undergoes, during its 

 transit through the Fallopian tube, have been investigated by Dr. Barry and 

 Professor Bischoff :f and, more recently, the analogous changes which occur 

 in the bitch's ovum have also been examined by Bischoff.J Dr. Barry's 

 earlier observations were given at page 1565 of Miiller's Physiology. His 

 later Researches afford results, which in respect to the more im- 



* M. Coste (Comptes Rendus, 5 Avril, 1847,) states, however, that he has observed the 

 process of cleaving in the ovum of birds during its passage along the oviduct; and he remarks 

 that it is limited to that portion of the yolk which constitutes the cicatricula. 



t Entwickelungs-geschichte des Kaninchen-eies, 1842. 



+ Entwickelungs-geschichte des Hunde-eies, 1845. 



Third Series, Philosophical Transactions, 1840. 



