IN MAMMALIA. "75 



portant and trustworthy points, are confirmatory of these earlier ones. 

 And if merely the figures which he gives be examined (see the figures 

 copied in Muller's Physiology, page 1567, and the plates in Dr. Barry's 

 second and third Series of Researches on Embryology),* it will be seen that 

 the appearances in the rabbits' ova taken from the Fallopian tube, agree in 

 the most important points with the account of the first type of the process 

 of cleaving observed by Bagge and Kb'lliker, in the ova of the three 

 varieties of Ascaris. For Dr. Barry not only saw the division and sub- 

 division of the yolk, but he also distinguished a pellucid space or nucleus 

 in each of the segments, and he seems to have observed the development 

 of two embryonic cells within the first single cell which takes the place of 

 the germinal vesicle in the yolk, the subsequent development of two 

 younger cells within each of these, and the continued repetition of this 

 mode of multiplication. 



The description of the process given by Bischoff, as he witnessed it in 

 the ova of the rabbit and of the bitch, is generally similar to Bagge's 

 account of what he observed in the Strongylus auricularis. When, in 

 these Mammalia, the ovum has passed the middle of the Fallopian tube in 

 its transit to the uterus, the yolk, which was previously one compact 

 uniform mass, begins to be resolved into a number of smaller spheroidal 

 masses ; first into two, then into four, then eight, then sixteen, and so on 

 (see fig. 9). Each segment contains a transparent vesicle, which is 

 seen with difficulty, especially in the bitch's ovum, on account of its 

 being enveloped by the yolk granules, which adhere closely to its 

 surface. This vesicle, Bischoff says, when liberated from the surrounding 

 yolk granules, most nearly resembles a fat or oil-globule. He has never 

 been able to detect a nucleus in it, though he has repeatedly and carefully 

 examined it for this purpose in the ova of the rabbit and the bitch. 

 Bischoff, therefore, cannot subscribe to the opinion, that the central vesicle 

 of each globular segment of the yolk is a nucleated, or embryonic, cell. 

 Neither does he regard the globular segments themselves as cells, for he 

 states that neither in the rabbit nor in the bitch can any investing mem- 

 brane be discerned. He considers them as simple aggregrations of yolk 

 substance around the central body or vesicle, such as the earlier divisions 

 of the ova of Ascarides nigrovenosa and acuminata and those of Sepia are, 

 according to Kolliker. 



It will be seen that the mode of division and subdivision of the yolk of 

 the mammal's ovum, agrees in its principal and more obvious features with 

 the first type described at page 66, as occurring in invertebrate animals. 

 The process cannot, however, be regarded as essentially the same in the 

 two cases, until the central vesicles of the segments in the mammal's 

 ovum shall be shewn to be nucleated cells, the nuclei of which multiply 

 by division, and thus determine the multiplication of the cells, two new 



* Philosophical Transactions, 1839 and 1840. 



