CHAPTER VIII 

 THE BLASTULA, GASTRULA, AND GERM LAYERS 



MORPHOGENETIC PROCESSES 



IN this chapter we shall endeavor to summarize the more 

 general processes of early development which lead to the 

 formation, out of the group of cleavage cells, of an embryo 

 possessing the beginnings of the chief organs or systems. This 

 will carry us from the formation of the blastula, through the 

 important events of gastrulation and germ layer formation, 

 and the varied processes by which tissue and organ rudiments 

 are set apart and differentiated. 



We shall give particular attention, indeed practically shall 

 limit ourselves, to a descriptive morphological account of 

 these events. This is done, not as a matter of choice, but 

 because the experimental results of the functional analysis 

 of these processes are so fragmentary and so scattered, that the 

 attempt at their summary, in a text of this character, seems 

 unwise. This is partly because the efforts to analyze these 

 processes experimentally have been delayed until the similar 

 problems of cell organization and cleavage should have been 

 more satisfactorily "solved. These topics we have already 

 considered. We should say, however, that what has already been 

 accomplished in the way of describing these later phenomena 

 of the mechanics of development (Entwicklungsmechanik, 

 Roux) bears out the general conclusions indicated by the earlier 

 processes, namely, that while the action of external conditions 

 as stimuli is essential, their place normally is chiefly that of 

 affording the general conditions of life and development. The 

 actual quality and the really significant details of the later, 

 as of the earlier phenomena of development and differentia- 

 tion, depend primarily upon internal conditions and relations. 



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