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Show 286 Mil. 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 2 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 3 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 intussusception 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 disintegration 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 I 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.S. 1880, p. 661). 2 " O n the Developmental History of Mollusca," Phil. Trans. 1875, p. 43. ;t " VVeitere Studien iiber d. Enlwickclungd. Ascidien," Archiv f. mikr. Anat Bd. vii. (1871) p. 101. |
