248 



outwards ; viewed from above, it has a central pellucid 

 area and a marginal opaque area. The pellucid area be- 

 comes pear-shaped, and in its narrow posterior third the 

 primitive streak appears lengthwise ; it develops a longi- 

 tudinal primitive groove. Ectoderm cells, which are de- 

 tached at the sides of the streak, form, along with lower- 

 layer cells, the mesoderm. In the front part of the pellucid 

 area, a bulging arises, the head fold (its endodermic cavity 

 is the fore gut) ; and between it and the streak, arise the 

 ectodermic ridges of the medullary plate, thus forming the 

 medullary groove. The ridges (medullary folds), continu- 

 ous in front, meet gradually backwards to form the neural 

 tube, beneath which a line of endoderm becomes the 

 notochord. 



The sheet of mesoderm at each side becomes two-layered ; 

 that next the ectoderm is the somatic layer, and that next 

 the endoderm is the splanchnic layer, the cavity between 

 being the coelome. Further, the mesoderm next the noto- 

 chord is thicker (vertebral plate), and it becomes marked 

 off from the outlying portion (lateral plate), and is also 

 split up into transverse segments (protovertebrae or myo- 

 tomes). The coelome of the myotomes is the myocoele, 

 and that of the lateral plates is the splanchnocoele. The 

 two layers (somatic mesoderm and ectoderm, i.e., somato- 

 pleure) external to the splanchnocoele form the body wall ; 

 and the two layers (splanchnic mesoderm and endoderm, 

 i.e., splanchnopleure) internal to it form the gut wall. 

 The main part (mesenteron) of the gut is formed by the 

 folding-off of the embryo, when the yolk becomes a yolk-sac 

 connected by a narrow stalk with the mid-gut. 



The rudiments of blood - vessels, which appear in the 

 mesoderm, give a mottled appearance to the inner rim of 

 the opaque area (vascular area). Two vessels beneath the 

 fore-gut fuse to form the tubular heart ; its posterior end 

 is formed by the union of two vitelline veins, and a Y-shaped 

 vein (anterior and posterior cardinal forming duct of Cuvier) 

 opens into it at each side. 



Around the front of the head there is a part of the blasto- 

 derm which, before the mesoderm extends into it, is termed 

 the pro-amnion ; there the roof of the splanchnocoele 

 bulges upwards at each side, and on each dome a whitish 

 line appears. These lines, which curve inwards and meet 

 in front of the pro-amnion, are the amnion folds ; arching 

 over the head they coalesce and grow backwards, uniting 

 with similar folds which arise around the tail and (later) 

 at the sides. The embryo is thus enveloped in a double 

 fold of somatopleure ; the inner layer of which (the true 



