CHAPTER XIII. 

 THE FOLLICLE AND THE PLACENTA. 



While Todarro seems to have observed the proliferation of the follicle 

 cells of salpa, Salensky is the discoverer of their most remarkable and 

 complicated relation to the developing embryo. 



His memoir (5) is the first great step towards a true insight into the 

 embryology of salpa, and it must always hold a most prominent place in 

 the literature of the subject. 



My own work has been rendered much more simple by Salensky's 

 account, which has aided me so much that I cannot but regret the hostile 

 attitude which I have been forced to assume towards the most charac- 

 teristic opinions of its author, although I believe that a critical review of 

 his own account of the facts will lead the reader to adopt my interpreta- 

 tion of them. 



SECTION 1. The Origin of the Follicle. 



At the time of fertilization the egg is inclosed in an epithelial capsule 

 of follicle cells, the history of which, as already described, pp. 214 and 

 225, is as follows : 



The germinal mass which is formed in the body of the young solitary 

 salpa, at the place where the proliferous stolon is afterwards developed, is, 

 at first, a compact mass of undifferentiated cells, as shown in Plate XX, 

 Fig. 6, at n, and more magnified in Plate XLI, Fig. 7. As the stolon 

 grows, this mass becomes elongated into a rod, as shown in Plate XVI, 

 Fig. 5. 



At first, cells in all parts of the germinal mass show signs of multi- 

 plication, but the peripheral ones soon become differentiated into an 

 epithelial layer, Plate XX, Fig. 2, m, Plate XXI, m, and Plate XXXI, 

 Figs. 4, 5, 6, 7, 8 and 9, and a central core of germ cells, which latter lose 

 the power of multiplication which is retained by the peripheral cells, and 

 become converted into eggs. As the bodies of the chain-salpse become 

 marked off in the walls of the stolon, the epithelial layer also grows 

 inwards, as shown in Plate XV, Fig. 2, m, Plate XXIII, Fig. 5, m, 



