GASTRULATION PROPER 113 



merited marginal zone (blastoporal lips) are in sharp contrast 

 with the surrounded vegetal pole yolk-laden cells. 



7. The future epiholy of the marginal zone, accompanying invo- 

 hition at all points, resulting in a continued reduction in the size 

 of the circumblastoporal lips and of the exposed yolk plug. The 

 margins of the yolk plug are at first round, then vertically oval, 

 and finally the lateral lips of the blastopore approach each other 

 to form a vertical slit as the yolk plug is closed over and disap- 

 pears within. 



8. The origin of the internal or second layer of cells, arising from 

 the involuted dorsal lip cells and collectively known as the 

 endoderm. This sheet of cells fans out within the embryo to give 

 rise very soon to a new cavity, the gastrocoel or archenteron. 

 Since these inturning cells give rise largely to the roof and lateral 

 walls of the archenteron, those parts of the cavity will be lined 

 with somewhat pigmented cells from the original epiblast. There 

 is a gradation or lessening of this pigment in the archenteric roof 

 cells as one progresses anteriorly in the gastrula. 



9. Simultaneously with the origin of the inner layer of endoderm, 

 some cells are proliferated o§ into the gastrular slit, between the 

 roof of the archenteron and the dorsal epiblast, which cells will 

 become the notochord. Before differentiation these cells are 

 called chorda-mesoderm. Since the lips of the blastopore are 

 circular, this proliferated mass of cells also becomes circular. 

 The more lateral and also the ventral proliferations become 

 mesoderm. There arises, therefore, the dorsal notochord and the 

 lateral and ventral mesoderm as a circle of tissue within the 

 fold of the blastoporal lips, occupying the space between the 

 epiblast and either the inner endoderm or yolk endoderm. 



By this time the student must have realized that gastrulation is a 

 highly integrated mosaic of motions which are purposeful in that 

 they achieve the state of the completed gastrula. Spemann aptly wrote: 



However harmonious the process of motion may be by which the material 

 for the chief organs arrives at the final place, and however accurately the 

 single movements may fit together . . . they are nevertheless no longer 

 causally connected, at least from the beginning of gastrulation onward. 

 Rather, each part has already previously had impressed upon it in some way 

 or other, direction and limitation of movement. The movements are regu- 

 lated, not in a coarse mechanical manner, through pressure and pull of 

 simple parts — but they are ordered according to a definite plan. After an 



