GENERAL EMBRYOLOGY 67 



blastopore and yolk plug. Bisect the gastrula by a cut through the yolk plug 

 and the center of the black hemisphere. Examine the cut surface, compare 

 with Holmes's Figs. 19 and 20 (pp. 94 and 96), and draw. The black cells are 

 the ectoderm, the white cells, the entoderm, and about this time a third layer, 

 the mesoderm, begins to grow out between the ectoderm and entoderm from the 

 cells around the blastopore. 



4. Origin of the nervous system; neural fold stage. Examine embryos of 

 this stage with a lens and note that a fold is appearing on each side of a central 

 groove extending lengthwise along the black hemisphere. This groove marks 

 the dorsal median line of the future embryo. The pair of folds, the medullary folds, 

 later come in contact and fuse in the median dorsal line, thus forming a longitudi- 

 nal tube which is the central nervous system. The blastopore is now reduced 

 to a small hole. Draw. Holmes's Fig. 22 (p. 98) is a cross-section of this stage. 



5. Mesoderm and coelome. Examine slide. This is a cross-section of a 

 stage about halfway between Holmes's Figs. 22 and 26. The cross-section is 

 roughly pear-shaped, the narrow end of the section being the dorsal side of the 

 embryo. The section is surrounded by a layer of uniform width, two or three 

 cells thick. This is the ectoderm, destined to become the epidermis of the skin. 

 In the dorsal median line just under the ectoderm is an oval mass of cells with a 

 central elongated cavity. This is the cross-section of the neural tube or central 

 nervous system, which, as has been seen, originates as a pair of ectodermal folds, 

 which then umte to form a tube, while the ectoderm fuses again over the tube 

 to a continuous layer. Just ventral to the tube is a circular mass, the notochord, 

 which arises by an upfolding of the dorsal wall of the intestine. It is a long, 

 slender rod, around which the vertebrae later develop. Ventral to the notochord 

 appears the primitive intestine, a large, nearly circular, mass of ill-defined cells. 

 Its dorsal wall is thin and overlies a cavity, the cavity of the future digestive 

 tract; its ventral wall is very thick on account of the yolk which it contains. 

 This intestine is the entoderm, and its cells become the lining epithelium of the 

 digestive tract. Between the ectoderm and the entoderm lies the mesoderm; 

 it consists of a large triangular mass of cells on each side of the neural tube from 

 which a layer of cells extends ventrally on either side of the intestine, meeting 

 below. The layer of mesoderm cells generally shows a central split, which is the 

 coelome. The mesoderm to the outer side of the split, next to the ectoderm, 

 then represents the parietal layer of the peritoneum and some of the connective 

 tissue of the body wall. The mesoderm on the inner side of the split, next 

 to the entoderm is destined to form the connective tissue and muscular layers 

 of the wall of the digestive tract, the visceral layer of peritoneum, and the 

 mesenteries. The masses of mesoderm at the sides of the neural tube and 

 notochord form the axial skeleton, nearly all of the voluntary muscles, and the 

 dermis of the skin. The mesoderm immediately ventral to the triangular mass 

 is the source of the urinogenital system. 



