1919.] T. SouTHWEiyiy & B. Prashad : Studies of Indian Fishes. 237 



also be borne in mind that though we have a nearly complete 

 series from small projections on the wall of the placental cord to 

 long thread-like appendicula, this does not give us any clue as to 

 the evolution of these structures ; nor does it indicate any relation- 

 ships between the various forms ; because in the species with the 

 best- developed appendicula the placenta is of the most primitive 

 and least evolved type and vice versa. Indeed, this last-stated 

 fact seems to show that the forms with a less highly organized 

 type of placenta requiring some other mode of absorption of food 

 have developed these additional structures. The appendicula, 

 if this is so, would be more of the nature of acquired or adaptive 

 structures than indications of any genetic relationships. 



Histological structure. — As seen in longitudinal sections (pi 

 xvii, fig. 8) the wall of the appendicula is found to be formed 

 of three to four layers of more or less potygonal cells ; the core of 

 the finger-shaped processes is filled up by loose connective tissue, 

 which reaches up to the walls of the channels in the placental 

 cord. In the connective tissue portion stellate cells can also be 

 distinguished here and there. 



Function.— Alcock, with some doubt, considered the appen- 

 dicula to be of the nature of lymphatic glands, provided the 

 channels of the placental cord be considered as lymphatices. 

 Their structure and various grades of development, and the blood 

 vessels in the appendicula of 5. palasorrah and S. sorrakowah, to- 

 gether with the grades of development of the placenta, tend to 

 show that they might, like villi, serve in absorbing the food 

 material secreted by the uterine wall of the mother. This secre- 

 tion, as was seen in the case of the specimens at Puri, surrounds 

 the embryos completely, just as the amniotic fluid does in the 

 mammals. 



Placenta. 



Having already dealt with the placental cord we will now 

 record a few observations about the placenta in some of the Indian 

 sharks that we have seen, besides adding some notes about the 

 forms previously described. 



It may be stated at the outset that the placenta in these forms 

 is of the nature of a yolk-sac placenta. When all the yolk in the 

 yolk-sac has been absorbed, nourishment must be obtained by the 

 embryo from the maternal uterus. This is done in a variety of 

 different ways. In the earlier stages in the aplacental forms the 

 branchial filaments are probably of use in absorbing the nutritous 

 secretions of the uterus in which the embryos are lying. Later on 

 special processes or trophonemata are developed from the uterine 

 wall and these, entering the spiracles of the foetus, pour the nutri- 

 tious secretion into the alimentary canal of the embryo. In the pla- 

 cental forms the yolk-sac is utilized for the formation of a placenta 

 and the connection of the yolk-stalk with the intestine becomes 

 obliterated ; the blood vessels on the other hand become specially 

 enlarged and nourishment is taken to the embryo directly through 



