224 MANUAL OF ANIMAL BIOLOGY 



(1) The first plane of cleavage begins near the center of the 

 animal pole and extends down through the vegetal pole, thus 

 cutting the egg into two equal sized cells, each composed of a 

 portion of both the animal and vegetal poles. (W. f. 174, A.) 



(2) The second cleavage is at right angles to the first, but in the 

 same direction, so that the embryo now consists of four similar 

 cells, each composed of material from both poles. ( W. f. 174, B.) 



(3) The plane of the third cleavage is at right angles to the 

 first and second. It cuts the quadrants, previously formed, in 

 the region which separates the animal pole from the vegetal pole. 

 The resulting eight cells consist of an upper quartet of small, dark- 

 colored cells formed from the animal pole, and a lower quartet of 

 large cells formed from the vegetal pole. (W. f. 174, C.) 



(4) The fourth and fifth cleavages are parallel to the first and 

 second, but between them. They both begin in the animal pole 

 and slowly extend through the vegetal pole. 



(5) The sixth cleavage is parallel to the third, but lies above 

 it and is, therefore, entirely in the animal pole. 



(6) After the sixth cleavage the divisions become more or less 

 irregular. As a result of the more rapid division of the dark- 

 colored cells of the animal pole, they are much smaller after a time 

 than those of the vegetal pole. (W. f. 174, D.) 



Blastula. At about the 24-cell stage definite indications are 

 found of an internal cavity, termed the segmentation cavity, or 

 blastocoel, situated well toward the animal pole of the egg. As 

 cleavage continues, this segmentation cavity, as shown by a study 

 of prepared sections, increases in size. It is enclosed above and on 

 the sides by layers of the dark cells, and, in the same way, below 

 by the larger white cells. Such a stage constitutes the blastula 

 of the Frog's egg, and it has the general external appearance of a 

 golf ball with approximately one-half of the surface (animal pole) 

 blackened. (W. f. 174, D, E, F.) 



Gastrula. Typically a gastrula is a two-layered sac which 

 develops from the one-layered blastula by invagination of a portion 

 of the outer ectoderm to form an inner endoderm layer. In the 

 Frog, however, owing to the large amount of yolk stored in the 

 vegetal pole, the invagination of the ectoderm is considerably 

 modified, and the two-layered gastrula stage is reached partly by 

 an overgrowth and partly by an invagination of the dark ectoderm 

 cells. The comparatively rapid growth and division of the dark 



