282 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



that optimum morphological conditions must be elaborated during the cleav- 

 age phase of development along with the developing physiology of the blastula. 



a. Morphological Relationships of the Blastula 



There are two aspects to the developing morphology of the blastula, namely, 

 the formation of the blastoderm and the blastocoel. 



During cleavage and blastulation, the structure of the blastoderm is elabo- 

 rated in such a manner that the major, presumptive, organ-forming areas of 



the future embryonic body are segregated into definite parts or districts of 

 the blastoderm. The exact pattern of arrangement of these presumptive, organ- 

 forming areas varies from species to species. Nevertheless, for a particular 

 species, they are arranged always according to the pattern prescribed for that 

 species. This pattern and arrangement of the major, presumptive, organ- 

 forming areas permit the ordered and symmetrical migration and rearrange- 

 ment of these areas during gastrulation. 



Similarly, the blastocoel is formed in relation to the blastoderm according 

 to a plan dictated by the developing mechanisms for the species. One of the 

 main functions of the blastocoel is to permit the migration and rearrangement 

 of the major, presumptive, organ-forming areas during gastrulation. Conse- 

 quently, at the end of the blastular period, the blastoderm and the blastocoel 

 are arranged and poised in relation to each other in such a balanced fashion 

 that the dramatic cell movements of gastrulation or the next period of devel- 

 opment may take place in an organized manner. 



b. Physiological Relationships of the Blastula 



The development of a normal-appearing, late blastula or beginning gastrula 

 in a morphological sense is no proof that proper, underlying, physiological 

 states have been established. A few examples will be given to illustrate this fact: 



1) Hybrid Crosses. When the sperm of the wood frog, Rana sylvatica, 

 are used to fertilize the eggs of the ordinary grass frog, Rana pipiens, cleavage 

 and blastulation appear normal. However, gastrulation is abortive, and the 

 embryo soon dies (Moore, '41, '46, '47). 



2) Artificial Parthenogenesis. In the case of many embryos, chordate and 

 non-chordate, in which the egg is stimulated to develop by means of artificial 

 activation, the end of the blastular stage may be reached, but gastrulative 

 processes do not function properly. A cessation of development often results. 



3) Oxygen-block Studies. In oxygen-block studies, where the fertilized eggs 

 of Rana pipiens are exposed to increased partial pressures of oxygen from 

 the time of fertilization to the four- or eight-cell stage, the following cleavages 

 and the morphology of the blastula may appear normal, but gastrulation does 

 not occur. Similar oxygen-pressure exposures during the late blastular and 

 early gastrular stages have no effect upon gastrulation. This fact suggests that 



