398 GASTRULATION 



cavity or into spaces or cavities developed within the developing body. In the 

 primitive-streak area of reptiles, birds, and mammals, for example, meso- 

 dermal cells detach themselves from the primitive streak and migrate into the 

 space between the epiblast and hypoblast. Also, in Hie formation of the two- 

 layered embryo in the prototherian mammal, Echidna, the inward migration 

 of small entodermal cells to form the hypoblast may be regarded as cellular 

 ingression (fig. 175D). Ingression and polyinvagination have similar meanings. 



g. Delamination 



The word delamination denotes a mass sunderance or separation of groups 

 of cells from other cell groups. The separation of notochordal, mesodermal, 

 and entodermal tissues from each other to form discrete cellular masses in 

 such forms as the teleost fish or the frog, after these materials have moved 

 to the inside during gastrulation, is an example of delamination (fig. 210E, F). 



h. Divergence 



This phenomenon is the opposite of convergence. For example, after cells 

 have involuted over the blastoporal lips during gastrulation, they migrate and 

 diverge to their future positions within the forming gastrula. This movement 

 particularly is true of the lateral plate and ventral mesoderm in the frog, or 

 of lateral plate and extra-embryonic mesoderm in the reptile, bird, or mammal 

 (fig. 192B, C, small arrows). 



/. Extension 



The elongation of the presumptive neural and epidermal areas externally 

 and of the notochordal, mesodermal, and entodermal materials after they have 

 moved inward beneath the neural plate and epidermal material are examples 

 of extension. The extension of cellular masses is a prominent factor in gas- 

 trulation in all Chordata from Amphioxiis to the Mammalia. In fact, as a 

 result of this tendency to extend or elongate on the part of the various cellular 

 groups, the entire gastrula, in many instances, begins to elongate in the antero- 

 posterior axis as gastrulation proceeds. The faculty for elongation and exten- 

 sion is a paramount influence in development of axiation in the gastrula and 

 later on in the development of primitive body form. The presumptive noto- 

 chordal material possesses great autonomous powers for extension, and hence, 

 during gastrulation it becomes extended into an elongated band of cells. 



D. The Organization Center and Its Relation to the Gastrulative Process 



1. The Organization Center and the Primary Organizer 



Using a transplantation technic on the beginning gastrula of the newt, it was 

 shown by Spemann ('18) and Spemann and Mangold ('24) that the dorsal- 

 lip region of the blastopore (that is, the chordamesoderm-entoderm cells in 

 this area), when transplanted to the epidermal area of another embryo of the 



