limit of neural plate 



mid and hind gut 



limit of head region (dashed line) 

 oral sucker, 

 lens 

 eye- 

 ear 

 notochord 



somite mesoderm 



anterior limb bud 



pharyngeal pouches 



foregut 



blastopore 



> ectode 



mesoderm 



entode 



B 



Figure 7-8. Outline of late frog blostulo with organ-forming areas of the surface outlined. 

 A, lateral view with future anterior end up; B, anterior view. (After Walter Vogt) 



called the proctodeum. The fusion and breaking through 

 between these two structures forms the anus. 



Anteriorly, a stomodeal groove has formed and breaks 

 through into the pharynx to form the mouth. A liver diver- 

 ticulum has penetrated into the yolk-ladened cells; a heart 

 has formed in the region below the pharynx and in front 

 of the liver. The neural tube is differentiating into the brain 

 at the late neurula stage. 



The development of the frog differs from that of the lam- 

 prey in that the mesoderm arises not as pouches or ridges 

 from the roof of the archenteron but as mesoderm essentially 

 separated from the entoderm during the process of involu- 

 tion at the dorsal lip. The mesoderm arises as bilateral 

 plates, which secondarily form segmental myotomes and 

 nonsegmcntal layers of splanchnic and somatic mesoderm 

 enclosing bilateral coeloms. These layers of mesoderm cover 

 the organs suspended in the coelom and the body wall re- 

 spectively. The medullary plates of these two types arise in 

 somewhat similar fashion, but that of the frog invaginates 

 to form a tube instead of sinking in as a solid strand. The 

 closure of the neural ridges begins anteriorly in the frog 

 rather than at the posterior end of the neurula. In both 

 Amphio.xus and the .frog, a proctodeum forms the anus, 

 whereas the lamprey retains the blastopore as this structure. 

 Both .Amphioxus and the frog have a neurenteric canal; 

 that of the frog is closed very early, while that of Am- 

 phioxus persists for some time. In the lamprey such a canal 

 is never formed. 



The development of the lamprey neural tube appears to 

 be specialized. That of the frog is a more primitive type of 

 tube, for it is a neurenteric canal. 



Chick The chicken egg is more complex structurally than 

 the previous types (Figure 7-1 D). It has a hard shell in- 

 side of which is an outer and inner shell membrane; the 

 two membranes are separated by a small air space at the 

 large end of the egg. Inside the inner shell membrane is a 



layer of albumen and inside this, the ovum consisting of a 

 yolk mass on top of which floats a small germinal disc of cyto- 

 plasm containing the nucleus. The ovum is enclosed in an 

 albuminous chalaza forming a spiral strand at either end, 

 and the whole floats in the albumen. 



The ovum is of an extreme telolecithal type; in fact, it 

 has been described as megalecithal. The egg is fertilized 

 before it is enclosed in the chalaza, albumen, and shell in 

 the oviduct. It is laid at an early gastrula stage, after about 

 24 hours of development. 



The germinal disc is all that undergoes cleavage. The 

 first cleavage plane is simply a groove in its surface (Figure 

 7-9); the second lies at right angles to this. From this stage, 

 the cells form a rather irregular and expanding pattern on 

 the surface. The free margin of the blastodisc moves out 

 laterally and gradually overgrows the yolk mass. Periblast 

 or incompletely separated yolk cells unite the blastodisc 

 with the yolk mass, particularly at its anterior end, thus 

 forming a zone of junction. 



As cleavage continues, the blastodisc becomes several cells 

 in thickness and separates from the yolk centroposteriorly 

 by the blastocoel. Below the center of the blastocoel, there 

 are no periblast cells. As the blastodisc expands, it develops 

 a distinct margin of overgrowth. It begins to thin in its pos- 

 terior region, becoming about two cell-layers thick. 



The margin of the blastodisc forms a pit-like blastopore 

 where the cells begin to involute. Along with involution, 

 there is also a certain amount of ingression of individual 

 cells toward the blastocoel surface. The involution and in- 

 gression increases the thickness of the blastodisc in this area, 

 which now delaminates anteriorly into an outer epiblast 

 and an inner hypoblast (Figure 7-9 G). The area overlying 

 the blastocoel is somewhat transparent and is called the 

 area opaca, while that part of the blastodisc in contact with 

 the yolk, the zone of junction, is called the area pellucida. 

 As the process of gastrulation continues, there is a distinct 

 movement of cells (concrescence) toward the blastopore 



CLEAVAGE, GASTRULATION, ORGANOGENESIS • 201 



