286 MR. F. E. BEDDARD 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 Sepia” 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 (Joc. cit. pl. 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 inte the interior of the ovary (oc. cit. 
pl. 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. pl. 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 
1 T imagine that few will dispute Prof. Huxley’s opinion that the Mammalia 
in the course of their evolution have passed through a Dipnoid stage (see 
P. Z.8. 1880, p. 661). 
* «On the Developmental History of Mollusca,” Phil. Trans. 1875, p. 43. 
* “ Weitere Studien iiber d. Entwickelung d. Ascidien,” Archiv f. mikr, Anat. 
Bd. vii. (1871) p. 101. 
