MODIFICATION OF INTESTINAL-GLANDULAR LAYER. 333 



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

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



mr' 



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

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

 tended on one side of the intestinalgerm- vesicle (kh). B. In the centre of the 

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

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

 glandular layer {del). D. Skin-fibrous layer (hf) 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 (wb). E. The intestintil 

 tube (mr) is closed; the body-cavity (c) begins to form. F. The primiti^e 

 vertebi'a3 (iv) appear ; the intestine (d) is almost completely closed. G. 

 The primitive vertebrae (10) begin to grow round the medullary tube (mr) 

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

 vesicle (nb). H. The vertebrae (iv) 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 ; lo, primitive vertebrae ; ch, notochord ; c, body- 

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

 testinal cavity ; nh, navel-vesicle. 



