DERIVATION OF BODY TUBE FROM EMBRYONIC DISC 13 



The dorso-ventral direction of development, together with the plate-Uke form of the embry- 

 onic disc, has a different phylogenetic significance. Both are probably inherited from an ances- 

 tral t-ype with a yolk-laden ovum. In such an ovum, with the meroblastic type of segmentation, 

 the flattened embryonic disc gradually spreads from the dorsal surface in a ventral direction 

 around the underlying yolk-mass. 



Derivation of body tube from embryonic disc. — The primary result of the 

 precocious growth in the dorsal region of the embryonic disc is the conversion of 

 the disc into the body tube, curved ventrally in its long axis (fig. 14). 



Fig. 15. — Portion of Cross Section of the Embryo shown in Fig. 13. ch, notochord. 

 d, somatic mesoderm, df, splanchnic mesoderm, g, junction of extra-embryonic somatic and 

 splanchnic mesoderm, ek, ectoderm, en, endoderm. me, embryonic mesoderm. /, neural 

 groove, p, beginning of embryonic ca?lom (pericardial cavity). (Minot, after Graf Spee.) 



^x:x^ 



-p 



As a result of the more rapid expansion of the germ layers (especially the ectoderm) near 

 the mid-Une, the dorsal sm-face of the embryonic disc in general becomes convex, with a depres- 

 sion laterally (where growth is less rapid) forming a groove at the hne of attachment of the am- 

 nion (figs. 11, 12, 13, 14 B). The unequal growth in the germ laj^ers is clearly evident m the cross 

 section shown in fig. 15. By a continuation of this process, the margins of the embryonic disc 

 become still further depressed and finally folded in ventrally so as to transform the disc into a 

 tube (fig. 14 D). Similarly, by a more rapid expansion of the dorsal layer of the disc in the lon- 

 gitudinal axis, the head and tail ends of the disc are folded and tucked in ventrally, and the 

 primitive body tube is thus correspondingly curved in its long axis (figs. 14 A, 14 C). 



Fig. 16. — Model of Human Embryo 1.8 mm. Long. Viewed from above, the roof of the 

 amniotic cavity having veen removed. Near the caudal end of the neural groove, the primitive 

 pit (opening of neurenteric canal) is visible. The primitive somites are appearing in the occip- 

 ital region, the fourth corresponding to the boundary between head and neck. (McMurrich, 

 from Keibel and Elze.) 



The embryonic disc is thus converted into a tube composed of an outer laj^er of ectoderm, 

 a middle layer of mesoderm and an innor layer of endoderm. The yolk-sac now presents an 

 expanded yolk-vesicle Uned by endoderm which is still continuous through the constricted yolk- 

 stalk with the endoderm lining the primitive enteric cavity (fig. 14 C). The enteric cavity (or 

 archenteron) has a bUnd tubular prolongation (fore gut) into the head region, and another (hind 

 gut) into the tail region. From the latter a slender diverticulum, the allantots, extends into the 

 body stalk (later the umbiUcal cord). The allantois is an organ of phylogenetic importance, 

 with which the urinary bladder is later connected. 



Formation of the neural tube.— The principle of unequal growth applies to the 

 formation not onlv of the body as a whole, but also of its constituent parts. 

 Thus the anlage of the nervous system arises from the ectoderm as a wide groove 



