970 HEAD-FOLD AND CAUDAL FOLD. 



ovum as derivatives of the epithelium of the Graafian follicle. His designates 

 these formations parablastic, in contradistinction to the archiblastic, which belong 

 to the three germinal layers of the embryonal rudiment. Waldeyer also believes 

 in the parablastic formation of blood, vascular end othelium, and connective tissue, 

 although he considers the material from which the latter are derived as cohesive, 

 and as living protoplasm of the same significance as the elements of the germ. 

 The doctrine of archiblast and parablast has recently experienced many modi- 

 fications. 



The development of the middle germinal layer and the formation of the organs 

 derived from it constitute one of the most difficult problems for investigation. 

 The work of recent investigators, particularly^ that of the brothers Hertwig, has 

 shown that in the lower vertebrates (amphioxus, triton), the chorda dorsalis 

 and both walls of the ccelom-cavity result from evaginations of the hypoblast, as 



/ II III 



FIG. 378. Scheme of the Formation of the Chorda and the Coelom through Evagination of the Hypoblast after the 



Theory of the Brothers Hertwig. 



Fig. 378 illustrates in a diagrammatic way. In I is the beginning of the central 

 evagination (for the chorda) ; the two lateral evaginations (for the walls of the 

 ccelom) are still in free communication with the hypoblast ; in // the points of the 

 evagination are narrowed ; and in /// the chorda (which now lies below the medul- 

 lary canal likewise constricted off) is fully detached and appears in cross-section as a 

 round body. In the same way the walls of the coelom-cavity have become de- 

 tached, and they exhibit their two plates, the somatopleure and the splanchno- 

 pleure, and between the two the large body-cavity has expanded. The intestinal 

 tube and the body-cavity have thus each obtained an independent wall. Accord- 

 ing to many new investigations both ectoderm and entoderm participate in the 

 formation of the mesoderm, which, in its turn, is capable of producing the most 

 varied tissues, with the exception of the nerves. 



FOLDING OFF OF THE EMBRYO. FORMATION OF THE HEART 

 AND THE FIRST CIRCULATION. 



Up to this time the embryo with its three germinal layers has occu- 

 pied the level of the layers themselves. Now (Fig. 376, V) the cephalic 

 portion raises itself above this level and, becoming free, it grows more 

 and more forward. There is thus formed in front of and under the head 

 an mvagination of the germinal layers known as the head-fold (V, r). 

 The prominent cephalic portion is hollow within and an entrance may be 

 gained from the interior of the germinal vesicle into the cephalic cavity. 

 The latter is designated the fore-gut cavity (V, D), and the entrance to 

 the anterior intestinal portal. The formation of the fore-gut through 

 the elevation of the head from the level of the germinal layers occurs in 

 the chick as early as the second day (in dogs on the twenty-second day). 

 In an entirely similar manner, although somewhat later (in the chick 

 the third day, in dogs on the twenty -fourth day), the analogous 

 tormation of the caudal portion takes place, and in consequence of which 



