336 COMPARATIVE ANATOMY 



New leucocytes are continually formed in the marrow of the 

 bones, as well as in the lymphatic glands and spleen ; the spleen 

 is apparently also of importance in absorbing the broken-down 

 remains of the red blood- corpuscles. 



MODIFICATIONS FOE, THE INTER-UTERINE NUTRI- 

 TION OF THE EMBRYO: FCETAL MEMBRANES. 



I. ANAMNIA. 



IN several Elasmobranchs the oviduct gives rise to glandular 

 villi which secrete a nutritive fluid, and in an Indian Ray (Ptero- 

 platea micrura) there are specially long glandular villiform pro- 

 cesses which extend in branches through the spiracles into the 

 pharynx of the embryos, of which there may be as many as three 

 in each oviduct. The gill-clefts of the embryos are in close appo- 

 sition, and there are no gill filaments (see p. 278). 



In certain viviparous Sharks (viz., Mustelus Isevis and Carcha- 

 rias) the walls of the vascular yolk-sac become raised into folds or 

 villi, which fit into corresponding depressions in the walls of the 

 oviduct, t'he latter becoming very vascular. A kind of umbilical 

 placenta is thus formed, by means of which an interchange of nutri- 

 tive, respiratory, and excretory matters can take place between the 

 maternal and foetal blood-vessels. 



Amongst viviparous Teleosts (comp. p. 360) various arrange- 

 ments for the nutrition of the embryo occur. In Zoarces 

 viviparus (and probably also in the Embiotocideae), the embryos 

 are retained in the hollow ovary, the empty follicles (corpora lutea) 

 of which give rise to extremely vascular villi, from which a serous 

 fluid containing blood- and lymph-cells is extruded into the cavity 

 of the ovary and thus surrounds the masses of embryos. These 

 swallow the fluid and digest the contained cells. In other forms 

 (e.g., viviparous Blennies, and Cyprinodonts), the embryos undergo 

 development within the vascular follicles, and are probably nour- 

 ished by diffusion; while in Anableps, villi are developed from the 

 yolk-sac, and these doubtless absorb the nutritive fluid from the 

 walls of the ovary. 



In certain Amphibians which have no prelarval existence, in- 

 teresting modifications occur for nourishing the young until the 

 larval stage is passed. Thus in the Alpine Salamander (Salaman- 

 dra atra), a large number of ova (40 60) pass into each oviduct, 

 just as in the allied S. maculosa, in which the young are born as 

 gilled larvaa. Were this the case in S. atra, the young would be 

 carried away in the mountain streams and destroyed, and a curious 

 adaptive modification has therefore arisen in this form, in 



