286 Mi;. V. E. BEODARD ON THE [May 4, 



occurrence. I would suggest myself that this migration of follicular 

 cells in the Mammalia is a record of what occurred in their Dipnoan 

 ancestors \ where the migration of follicular cells into the interior of 

 the ovum was an important process in connection with the formation 

 of yolk. 



It is, however, among the Invertebrata that the most striking 

 resemblances to the immigration of follicular cells in Lepidosiren are 

 to be found, and more particularly in the Cephalopoda. 



Some remarkable facts with respect to the nutrition of the ova of 

 Sepia have been discovered by Lankester, which are analogous to the 

 facts recorded by myself in Lepidosiren. In Sejna ^ the ova are 

 surrounded by a single layer of follicular cells which are supplied 

 externally with an abundant vascular network. The epithelial 

 layer becomes folded in a complicated fashion, and the folds project 

 into the interior of the ovum {loc. cit. pi. 12. fig. 23); the cells 

 of the epithelium take on a character resembling that of the goblet- 

 cells, and pour out their contents into the ovum ; moreover the cells 

 also proliferate and pass off into the interior of the ovary {loc. cit. 

 pi. 12. fig. 25), becoming gradually absorbed. 



Kowalevsky ^ has described a proliferation of the follicular cells of 

 Ascidians which migrate into the interior of the ovum, and eventually 

 form (loc. cit. pi. 10. figs. 2-4) a complete layer round it, so that 

 the follicle comes to be two layers deep ; the outer layer, which is 

 made up of very much larger cells, represents the original follicular 

 epithelium, while the inner layer consists of very much smaller cells. 



In many of the lower Invertebrata the ovum is largely nourished 

 at the expense of the surrounding cells. In the Platyhelminthes a 

 peculiar organ, the vitellarium, elaborates yolk-particles which are 

 subsequently absorbed by the ovum ; analogous processes take place 

 in many Insects. In all these cases the ovum retains the capacity, 

 inherited from its Protozoan ancestor, of feeding by the intussus- 

 ception of solid particles. 



All these facts seem to me to be directly comparable to what has 

 been described above in Lepidosiren. The secretion of yolk by the 

 follicular cells in the Platyhelminth and the absorption of this yolk 

 by the ovum only differs in degree from the proliferation of the 

 follicular cells in Sepia and Lepidosiren and their subsequent 

 absorption by the ovum. 



I may say a few words with regard to the special fact that the 

 yolk is largely elaborated with follicular cells and conveyed to the 

 ovum by the proliferation of these cells and their subsequent disinte- 

 gration in the interior of the ovum. Such a formation of yolk in 

 the Vertebrate ovum has not been put beyond a doubt ; indeed most 

 observations on the subject seem to show that the yolk is generally 



' I impgine tliat few will disfiute Prof. Huxley's opinion that the Mammalia 

 in the course of their evolution have passed through a Dipuoid stage (see 

 P.Z.S. 1880, p. 661). 



- " On the Developmental History of Molhisca," Pbil. Trans. 1875, p. -43. 



^ " Weitere Studien iiber d. Enlwickelung d . Asmlien," Arcbiv f. uiikr. Auat. 

 i;d. vii. (1871) p. 101. 



