CLEAVAGE 



225 



organism are also separate and remain so through their descend- 

 ants, the oogonia and oocytes, or spermatogonia and spermato- 

 cytes, of the mature individuals, until their period of synapsis, 

 when the members of each pair of chromosomes, similar but of 

 diverse ancestry, unite forming a single bivalent chromosome 

 which is represented in the mature ovum or spermatozoon 

 finally formed. It has already been suggested that this process 

 of synapsis, the ultimate fusion of paternal and maternal chro- 



FIG. 107. A. Cleavage figure in one of the first two blastomeres of the egg of 

 the Crustacean, Cyclops strenuus, showing the independence of the paternal and 

 maternal chromosome groups. After Riickert. B, C, D. Primitive germ cells 

 from embryos of the skate, Raja, showing the duplex character of the nuclei. 

 B and C are from a stage about the close of gastrulation ; D from a larva of 10 mm. 

 After Beard. 



mosomes, may be regarded as the final step in syngamy, and 

 that it is at the same time the first step in the beginning of a 

 new organism. 



With these briefly stated introductory facts in mind we may 

 proceed to a more precise description of the events of the 

 cleavage period. The process of cleavage may be described in 

 several different ways, or rather from several different view- 

 points. We may group these all under two heads and consider 

 cleavage, first, as a morphological process, emphasizing pri- 

 marily the forms of cleavage and describing the relation of the 

 cleavage planes and the blastomeres (a) to the entire zygote, and 



