CHAPTER IV 

 THE ESTABLISHMENT OF THE ENTODERM 



The morula stage; the formation of the blastula; the 



EFFECT OF YOLK ON GASTRULATION; GASTRULATION IN 

 BIRDS. 



The Morula Stage. — It should by no means be inferred that 

 cell division ceases with the cleavage divisions. The end of the 

 segmentation stage is not marked by even a retardation in the 

 succession of mitoses. Segmentation is regarded as ending when 

 the progress of development ceases to be indicated merely by 

 increases in the number of cells, and begins to involve localized 

 aggregation and differentiation of various groups of cells. 

 Development progresses from phase to phase without abrupt 

 ^change or interruption. The nomenclature and limitation of 

 the various phases of development are largely arbitrary and the 

 use of terms designating phases or stages of development should 

 not be allowed to obscure the fact that the whole process is a 

 continuous one. 



In eggs without a large amount of yolk, segmentation results 

 in the formation of a rounded, closely packed mass of blasto- 

 . meres. This is known as a morula from its resemblance to the 

 mulberry fruit which is in form much like the more familiar 

 raspberry or blackberry. At the end of segmentation the 

 chick embryo has arrived at a stage which corresponds with the 

 morula stage of forms with less yolk. It consists of a disc- 

 shaped mass of cells several strata in thickness, the blastoderm, 

 lying closely applied to the yolk. In the center of the blasto- 

 derm the cells are smaller and completely defined; at the per- 

 iphery the cells are flattened, larger in surface extent, and are 

 not walled off from the yolk beneath. 



The Formation of the Blastula. — The morula condition is of 

 short duration. Almost as soon as it is established there begins 

 a rearrangement of the cells presaging the formation of the 

 blastula. A cavity is formed beneath the blastoderm by the 



