246 GENERAL ZOOLOGY 



to have the haploid number of chromosomes, if it may be taken 

 for granted that the second maturation division is completed. 

 As a matter of fact, many of the larvae produced this way are 

 haploid in chromosome number; but in addition others are 

 diploid and a few triploid and tetraploid. How chromosome 

 numbers greater than the haploid are produced in these embryos 

 is a matter of speculation, but it is interesting that only diploid 

 animals metamorphose into frogs of both sexes. It would seem 

 that in the frog the diploid complement of chromosomes is neces- 

 sary for normal development. 



Cleavage.- — Cleavage is a period in development following 

 fertilization, marked by rapid mitotic cell divisions, in each of 

 which the diploid number of chromosomes is present. An excep- 

 tion to this so far as the chromosomes are concerned is in cases of 

 chromatin diminution, such as occurs in Ascaris, referred to in 

 a previous chapter; but in the majority of animals it is generally 

 believed that the diploid number of chromosomes appears on each 

 cleavage spindle. This seems to be the case in the frog. The 

 cells produced in the course of cleavage are known as blastomeres. 

 Some of these produce primordial germ cells, which later partici- 

 pate in gametogenesis, and the remainder become somatic cells. 



The manner in which cleavage takes place depends upon the 

 quantity and distribution of the yolk in the egg. The frog's egg 

 is called a telolecithal type of egg because the yolk is concentrated 

 about one pole of the egg. The first cleavage plane (the plane 

 separating the first two blastomeres) passes through the principal 

 axis of the egg, and therefore through a meridional plane, and 

 bisects the gray crescent. The furrow separating the blastomeres 

 begins in the animal pole and gradually cuts through to the 

 vegetal pole, forming two complete cells adhering to each other 

 on their adjacent faces (Fig. 152). Since the entire egg is 

 divided, the cleavage is spoken of as holoblastic. In contrast to 

 this, in a meroblastic cleavage, such as takes place in the hen's 

 egg, cleavage is confined to a small yolk-free area at the animal 

 pole, the yolk remaining undivided. In the frog the first cleavage 

 plane in the majority of cases coincides with the median plane 

 of the future embryo. In such cases the first two blastomeres 

 represent the right and left sides of the embryo. The second 

 cleavage plane is also meridional, at right angles to the first. The 

 third cleavage plane passes about 60 degrees from the animal 



