226 



Embryogenesis: Preparatory Phases 



up its rim, eventually closing to form a 

 small spherical invaginated gastrula (see 

 Fig. 74). These experiments lead Moore and 

 Burt to the conclusion that the forces of 

 gastrulation do not act in a median plane 

 through the poles of the embryo, but in 

 the plane of the entodermal plate at right 

 angles to the animal-vegetal axis. This 

 gastrula plate consists of two concentric 

 zones (see Fig. 73, left). The central spot (a) 

 is the first region to show activity, while 

 the remainder of the plate (p), which forms 

 a ring about a, is only secondarily involved 

 in the initial stages of the process. As soon as 



pieces of the entodermal plate of echino- 

 derm early gastrulae were imsuccessful be- 

 cause the cut edges showed a tendency to 

 spring apart, leaving an open sector. Moore 

 suggests that the whole entodermal plate is 

 normally under tension, and acts as one 

 concentric unit. 



Moore's own proposal that asymmetri- 

 cally (or excentrically) placed cytoplasmic 

 bridges between the cells could account 

 for gastrulation likewise appears to be in- 

 adequate, since such cell bridges by con- 

 tracting would tend only to draw cell 

 boundaries closer together and would not 



^ 



Fig. 74. Left: Diagrams illustrating the positions assumed successively by the gastral plate of Patiria 

 miniata during the hour following excision. Horizontal view. 



Right: Diagram illustrating the relations of the two planes considered in theories of gastrulation. fg. The 

 plane of the gastral plate, at right angles to the plane passing through the animal and vegetal poles, hi. (Fig- 

 ures from Moore and Burt, '39.) 



a has moved in to form a cone, p (the 

 plastic zone ring) begins to participate ac- 

 tively and moves toward the interior. A 

 wave of activity is then transmitted to a 

 ring of cells immediately below p (the ring 

 r), and these cells take up the work of 

 pushing the archenteric tube inward until 

 gastrulation is complete. It is the region p 

 which determines the subsequent fate of the 

 gastrula, i.e., whether it is to be normal or 

 an exogastrula. The cells of region r must 

 change from moment to moment as the 

 archenteric tube lengthens and the ento- 

 dermal cells become incorporated into it as 

 integral elements. In gastrulation, the cells 

 of a have the general form (in section) 

 of truncated cones. 



Glaser ('14) mentions an experiment in 

 which W. Roux cut out pieces of the early 

 neurula of the frog embryo. These detached 

 fragments, obtained by transverse section, 

 continued to infold. Moore's attempts to 

 repeat such results on isolated radially cut 



change the shape of cells unless new con- 

 nections were made as suggested in the 

 "zipper" hypothesis of Schmitt ('41). 



In Herbst's original lithium cultures, cer- 

 tain incomplete exogastrulae were described. 

 In these, the outer part of the gastrula plate 

 had evaginated, while the center had in- 

 vaginated. Moore takes these incomplete 

 exogastrulae as evidence that the central spot 

 of the entodermal plate is either more stable 

 or determined considerably earlier than the 

 adjacent plastic ring. 



Experimentally, it has been possible to 

 suppress invagination osmotically with a 

 non-penetrating sucrose solution, and on 

 the basis of such experiments an attempt has 

 been made to calculate the magnitude of 

 the pressure exerted normally by the in- 

 vaginating cone of the gastrula plate. The 

 figure Moore ('41, '45) gives is a pressure 

 of 2.6 to 5.5 mg., acting on a cross sectional 

 area of 707 square microns, or about 3.88 

 to 7.75 gm. per square millimeter. This fig- 



