DEVELOPMENT OF UNGULATA. 783 



disappears. A mass of albuminoid matter accumulates around the 

 ovum, as in Cavia, but is whiter in colour. It affords material 

 for imbibition, and the germ-mass becoming fluid at, or getting 

 fluid in, the centre, expands into a hollow sphere, the parietes of 

 which become differentiated into two layers : the outer one seems 

 to answer to the corresponding lamina demonstrated by Hunter 

 in the germinal area of the chick ; the other to the inner lamina 

 of the same area. 1 Both layers consist of coherent cells, with some 

 difference as to size and proportion of oil -globules. 



The ovum now grows rapidly in length, its opposite poles being 

 prolonged and attenuated. At the point where the two layers or 

 * membranes ' cohere, the embryonal trace appears, its long axis 

 extending at right angles to that of the ovum ; the inner or 

 ' mucous ' layer is so continued from the margin of the abdominal 

 depression as to ( appear of itself to form the intestine, existing 

 prior to that part being visible.' 2 The cephalic expansion and 

 incurvation seems relatively late in Ungulates. Before the amnios 

 is complete, and when the back of the embryo is still covered by 

 the peripheral part of the serous layer, when but three pairs of 

 pro to- vertebral nuclei are formed, and the cephalic ends of the 

 myelonal cords are only beginning to diverge, the opposite end of 

 the embryo begins to bud out two processes at right angles to its 

 axis, which soon expand into the allantois. 3 This vesicle rapidly 

 extends between the serous or animal layer forming the outer 

 coat of the ovum, on the one hand, and the embryo, amnion and 

 vitellicle, on the other hand ; carrying with it allantoic vessels, 

 and becoming coextensive with the outer coat. With this ex- 

 pansion that outer coat disappears, and the chorion is now repre- 

 sented by the vascular layer of the allantois itself, which has 

 become distinct from its inner or mucous layer. Meanwhile the 

 vitellicle has shrunk to slender proportions, its communication 

 with the intestine being reduced by growth of the abdominal 

 walls, and drawn out into an omphalo-mesenteric duct. The 

 vascular layer of the allantois, representing the chorion, effects 

 its vascular intus-susceptive relations with the uterine lining in 

 various ways. In most Perisso- and a few Artio- dactyles short 

 villous processes bud out from a greater part of the superficies of 

 the chorion, and a co-extensive minutely alveolar growth of the 

 lining- substance of the uterus receives them. 



Fig. 574 represents the foetal membranes and appendages 



1 xx. vol. v. p. 20, pi. 59, fig. 7. Later German Embryologists have called the one 

 1 serous ' or ' animal ' layer, the other ' mucous,' ' vegetal ' or ' organic' layer ; but any 

 of these terms can only be understood in an arbitrary sense. See Vol. II. p. 259, fig. 133. 

 2 lb. p. 20. 3 cclxiii". p. 18 (in the Roe-buck). 



