GASTRULATION IN VARIOUS CHORDATA 407 



whose midportion is located just above the future dorsal lip of the early gas- 

 trula, while the mesoderm lies to each side of the notochordal cells, extending 

 along the margin of the entoderm toward the corresponding mesodermal zone 

 of the other side (fig. 169D, F). The presumptive neural crescent occupies 

 a region just dorsal and anterior to the notochordal area. The remainder of 

 the animal pole is composed of presumptive epidermis. The presumptive 

 notochordal, neural plate, and epidermal areas are oriented along the general 

 direction of the future antero-posterior embryonic axis, the notochordal tissue 

 being the more posterior. Moreover, the midregion of the notochordal and 

 neural crescents at this time lies at the dorsal region of the future embryo 

 (fig. 194A). The presumptive entodermal area, on the other" hand, does not 

 have the same orientation as that of the above areas. In contrast, its axiation 

 is at right angles to the future embryonic axis (fig. 194A). If one views a 

 very early gastrula of the anuran amphibian in such a way that the beginning 

 blastoporal lip is toward the right (fig. 194A), then: 



( 1 ) The foregut material lies toward the right at the region of the forming 

 blastoporal lip; 



(2) the stomach material is slightly to the left of this area; and 



(3) the future intestinal area lies to the left and toward the vegetal pole. 



Therefore, one aspect of the gastrulative processes is to bring the ento- 

 dermal area into harmony with the future embryonic axis and, in doing so, 

 to align its specific, organ-forming subareas along the antero-posterior axis 

 of the embryo. In other words, the entodermal material must be revolved 

 about 90 degrees in a counterclockwise direction from the initial position 

 occupied at the beginning of gastrulation (compare fig. I94A, B). 



2) Physiological Changes Which Occur in the Presumptive Organ-forming 

 Areas of the Late Blastula and Early Gastrula as Gastrulation Progresses. 

 A striking physiological change is consummated in the presumptive organ- 

 forming areas of the epiblastic portion of the late blastula during the process 

 of gastrulation. This change has been demonstrated by transplantation ex- 

 periments. For example, if presumptive epidermis of the very late blastula 

 and early gastrula is transplanted by means of a micropipette to the pre- 

 sumptive neural area and vice versa, the material which would have formed 

 epidermis will form neural tissue, and presumptive neural cells will form epi- 

 dermis (fig. 196C, D). (See Spemann, '18, '21; Mangold, '28.) 



The experiment pictured in figure 196 involves interchanges between two 

 presumptive areas within the same potential germ layer, i.e., ectoderm. How- 

 ever, Mangold ('23) demonstrated that presumptive epidermis transplanted 

 into the dorsal-lip area, i.e., into the presumptive mesodermal area, may in- 

 vaginate and form mesodermal tissue. The converse of this experiment was 

 performed by Lopaschov ('35) who found that presumptive mesoderm from 

 the region of the blastoporal lip transplanted to the neural plate area of a 



