CHAPTER SIX 

 Blastulation 



Internally the 8 -cell stage of the frog shows the beginning of a 

 cavity where the well-formed animal hemisphere cells have completed 

 their 6-sided cell membrane. Once a blastomere is formed it tends to 

 acquire a certain degree of rigidity and will maintain its spherical cell 

 boundaries independently of its neighbors. Cortical and vitelline mem- 

 brane pressure may flatten cells slightly against each other. When seg- 

 mentation of the egg occurs, dividing it into smaller and smaller 

 cellular units, there naturally follows the appearance of an internal 

 cavity known as the segmentation cavity or blastocoel. The aggregate 

 becomes about 20 per cent larger than the total volume of the cells. 

 The earlier cleavages (described above) tend to be perpendicular to 

 the egg surface. However, after the 32-cell stage there appear division 

 planes more or less parallel to the egg surface, cutting off surface 

 protoplasm from inner protoplasm and yolk. With increased cell 

 rigidity, following each division, these surface cells push against each 

 other until they are lifted up and away from the underlying cells. A 

 crude simile would be to hold four or more marbles tightly together in 

 the hand, and no matter how much pressure is exerted there will always 

 be a space in the center of the group of marbles (i.e., cells). In the 

 frog's egg the formation of complete cells is rather slow, and the 

 blastocoel is small and not readily apparent until about the 32-cell 

 stage. 



The blastocoel is therefore a cavity, and this stage of development 

 is known as the blastula, overlapping the stages of cleavage. The cavity 

 is enlarged with each of the early cleavages and it is filled with an 

 albuminous fluid, arising from the surrounding cells. Even though the 

 frog's egg is telolecithal, cleavage has been described as holoblastic 

 (opposed to meroblastic) and the presence of this cavity distinguishes 

 it as a coeloblastula (as opposed to stereoblastula which has no 

 cavity). Since the horizontal cleavages appear toward the animal 

 hemisphere, this newly forming blastocoelic cavity will appear in an 

 eccentric position above the level of the equator, and slightly toward 



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