436 COMPARATIVE ANATOMY 



(Hatteria) : thus it is often situated near the stomach, but may 

 be met with in other regions of the intestinal tract, as, for 

 instance, at the commencement of the rectum (Anura, Chelonia). 

 In some cases (e.g. Sharks) it is broken up into several portions, 

 and frequently a larger " main spleen " and several smaller 

 " accessory spleens " can be recognised. 



The structures known as tonsils are most highly developed in 

 Mammals, in which they give rise to a paired organ lying on either 

 side of the fauces that is, in the region where the mouth passes 

 into the pharynx, and usually also to a mass situated more 

 posteriorly on the walls of the pharynx itself (pharyngeal tonsils) ; 

 the latter are phylogenetically the older organs and are present in 

 Reptiles, Birds, and most Mammals, and tonsil-like organs also 

 occur in Amphibians on the roof and floor of the oral cavity. 

 The tonsils consist of a retiform (adenoid) connective tissue 

 ground-substance enclosing a number of lymph-corpuscles, which 

 are arranged in so-called follicles. 



MODIFICATIONS FOR THE INTRA-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. In certain viviparous Sharks (viz., Mustelus 

 Isevis and Carcharias) the walls of the vascular yolk-sac become 

 raised into folds or villi, which fit into corresponding depressions in 

 the walls of the oviduct, the latter becoming very vascular. A 

 kind of umbilical placenta is thus formed, by means of which an 

 interchange of nutritive, respiratory, and excretory matters can 

 take place between the maternal and foetal blood-vessels. 



Amongst viviparous Teleosts various arrangements for 

 the nutrition of the embryo occur. In Zoarces viviparus and 

 in the Embiotocidea3 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 ovarian follicles, and are probably nourished by diffusion ; 

 in Anableps, villi are developed from the yolk-sac, and these 

 doubtless absorb the nutritive fluid from the walls of the ovary. 



