278 OUTLINES OF CHORDATE DEVELOPMENT 



and where the extra-embryonic portions of the two primary 

 germ layers are being differentiated. 



In enumerating the structures of the embryo itself (Fig. 107) 

 we may begin posteriorly and pass forward, since, read in this 

 direction, a series of transverse sections through a single embryo 

 roughly represents the steps in the development of most of the 

 parts fully formed at this time. Posteriorly there is a short 

 primitive streak region with thickened neural plate and primi- 

 tive groove. Passing anteriorly the neural folds rise and meet 

 forming the neural tube; the notochord becomes distinguishable, 

 and the mesoderm is differentiated into vertebral and lateral 

 plates connected by the intermediate cell mass; the lateral plate 

 is separated into somatic and splanchnic layers by the extra- 

 embryonic coelom. In the trunk and hinder part of the head 

 ten to twelve pairs of mesodermal somites are formed. 



The long head region extends forward from about the fourth 

 pair of somites (counting from the anterior end of the series). 

 The head is now almost completely separated from the yolk 

 ventrally, by the endodermal head-fold, while the ectodermal 

 head-fold, which also separates the lateral surfaces of the head 

 from the yolk and blastoderm, has more than half this extent. 

 The brain is the most prominent feature of the embryo, and its 

 primary divisions, fore-, mid-, and hind-brain, are well marked. 

 Neural crests are distinct in sections from the mid-brain poste- 

 riorly. The fore-gut, widely open posteriorly upon the yolk by 

 way of the anterior intestinal portal, is the only strictly endo- 

 dermal structure thus far differentiated. This is the pharyngeal 

 region, and the rudiments of the first or hyomandibular pair of 

 gill pouches are indicated laterally, while in its floor is the oral 

 plate. 



The large vitelline veins unite in front of the level of the first 

 somites, forming the heart, which now has both endothelial and 

 muscular layers. The elongated heart lies in the pericardia! 

 cavity and is bent to the right. Anteriorly it continues into 

 the diverging ventral aortaB, which pass around the sides of the 

 head, dorsally, as the first or mandibular pair of aortic arches; 

 these are continuous with the wide lateral dorsal aortse which 



