188G.] OVUM OF LEPIDOSIREN. 285 



much like that of the wliite yolk-spheres of Sauropsida. In any 

 case these bodies are not supposed to have an extrinsic origin, but 

 to arise within the ovum. On the other hand, a penetration of 

 follicular cells through the micropyle {loc. cit. pi. i. fig. 6) appears 

 really to occur in many osseous fish and to be comparable to the proli- 

 feration into the ovum of the follicular cells in Lepidosiren. 



Kolessuikow ' confirms the accuracy of His's results as to the 

 entrance of leucocytes into the ovum, but does not think this 

 process to have much functional importance. 



In Mammals a number of observations have been published which 

 tend to show that there is a migration of cells, which is evi- 

 dently comparable to the facts wliich I have detailed above in 

 Lepidosiren. Lindgren ^ has described such a migration of follicular 

 cells, and figures an ovum which is half filled with unaltered follicular 

 cells. Von Sehlen ^ and H. Virchow* have confirmed the accuracy 

 of Lindgren's observations. More recently Schiiter'* has described a 

 remarkable series of changes in the Rabbit's ovum which do not 

 altogether tend to the same conclusion. In young ova, which are 

 as yet surrounded by a follicle consisting of only a single layer of 

 cells, peculiar cells make their ajipearance in the peripheral regions, 

 and ultimately form a single layer of cells which surround the ovum, 

 lying beneath the follicular layer. Schafer believes that these ceils 

 are not derived from the follicular layer, but they originate in the 

 ovum. He compares very justly his own observations with those 

 of Kuppfer on Ascidia canina. Kuppfer*^ had shown that cells appear 

 in the interior of the ovum and range themselves round its periphery. 

 Kuppfer, however, believed that these cells originate in the ovum 

 itself, and are not, as Kowalevsky supposed, a product of the follicular 

 epithelium. His statements therefore are in complete accord with 

 those of Schafer ; while Lindgren, von Sehlen, and H. Virchow 

 describe a process in the maturation of the mammalian ovum 

 which is more comparable to that described by Kowalevsky in the 

 case of the Ascidians. The latest writer on the mammalian ovum, 

 Mr. Heape, did not find any such migration of follicular cells, and 

 concludes that the observations put on record by Lindgren, von 

 Sehlen, and Virchow are based upon abnormal processes. It is to 

 be noted, however, that the type studied by Heape'' (Mole) was not 

 studied by any one of these naturalists, and this fact may possibly 

 account for the discrepancies in their statements. In the latest 

 edition of Quain's 'Anatomy' it is suggested that the entrance of 

 follicular cells into the ovum described by Lindgren, von Sehlen, 

 and H. Virchow may be an abnormal process and not a regular 



1 Arcbiv f. mikr. Anat. vol. xv. (1878) p. 399. 



2 Arch. f. Anat. u. Phys. 1877. 

 ^ Arch. f. Anat. u. Phys. 1882. 



■* Arch. f. mikr. Anat. Bd. xxiv. (1884). 



5 Proc. Eoy. Soe. vol. xsx. (1880) p. 243. 



" Arch. f. mikr. Anat. Bd. viii. See also the papers of many others (Sabatier, 

 Eoule, Fol) on the Ascidian ovimi, and the facts referred to iu footnote on 

 p. 276, supra. These have been lately summed up in Quart. Journ. Micr. Sci., 

 June 188e, by Mr. Arthur Thomson. 



' Quart. Journ, Micr. Sci., Feb. 1886. 



