MODIFICATION OF INTESTINAL-GLANDULAR LAYER. 333 



simple globule, the wall of wliich consists of a simple layer 

 of homogeneous cells (Fig. 100, A del). The first change in 



Fig. 100. — The separation of the discoidal mammalian germ from the 

 yelk-sac, seen in section (diagrammatic). A. The germ-disc (/(, hf) lies ex- 

 tended on one side of the in testinalgerm -vesicle (kb). B. In the centre of the 

 germ-disc the medullary furrow (mr), and under that the notochord (ch) 

 appear. C. The intestinal- fibrous layer (elf) has grown round the intestinal- 

 glandular layer (dd). D. Skin-fibrous layer (/)/) and intestinal-fibrous layer 

 (df) part round the circumference of the germ-disc ; the intestine (d) begins 

 to separate itself from the yelk-sac or navel-vesicle (nh). E. The intestinal 

 tube (m?-) is closed ; the body-cavity (c) begins to form. F. The primitive 

 vertebrae (iv) appear ; the intestine (d) is almost completely closed. G. 

 The primitive vertebrae (u;) begin to grow round the medullary tube (mr) 

 and the notochord (ch) ; the intestine (d) is separated from the navel- 

 vesicle (nh). H. The vertebrae (w) have enclosed the medullary tube (mr) 

 and the notochord (ch) ; the body-cavity (c) is closed ; the navel-vesicle has 

 disappeared. The amnion and serous membranes are omitted. 



In all, the letters indicate the same parts : h, horn-plate ; mr, medullary 

 tube ; hf, skin-fibrous layer ; w, primitive vertebrae ; ch, notochord ; c, body- 

 cavity ; df, intestinal-fibrous layer • dd, intestinal-glandular layer ; d, in- 

 testinal cavity ; nh, navel-vesicle. 



