228 



GENERAL EMBRYOLOGY 



various forms of cleavage; the details are not particularly in- 

 structive and may be omitted. 



We may now turn to the morphological description of the 

 relations of the blastomeres among themselves. Before 

 describing the various relations which these may exhibit, it will 

 be useful to describe briefly a simple form of total and equal 

 cleavage, which may be regarded as a typical form. Such a 

 form of cleavage is indeed rare but it is found in the homoleci- 

 thal and holoblastic egg of the sea-cucumber, Synapta, as 



FIG. 108. Cleavage in the Holothurian, Synapta, Slightly schematized. 

 From Wilson, "Cell," after Selenka. A-E. Two-, four-, eight-, sixteen-, and 

 thirty-two cell stages. F. Blastula of 128 cells. B, in polar view, others in side 

 view. 



described by Selenka (Fig. 108). The earlier cleavage planes 

 always appear in a definite relation to the polar structure of the 

 ovum, and they are described as if the main axis of the egg were 

 in a vertical position. The first cleavage plane passes through 

 both poles and the chief axis of the egg, dividing it equally and 

 appearing on the surface as a complete meridian. The second 

 plane is similarly meridional or vertical, is at right angles to the 

 first, and divides the egg into four equal quadrants. The third 

 division plane is at right angles to the first two and is therefore 

 horizontal. In Synapta it is practically midway between the 



