CYTOLOGY OF REPRODUCTION IN ANIMALS 127 



The second important effect of syngamy is diplosis, the doubling of 

 the number of chromosomes by the union of the two gametic nuclei. 

 In syngamy two genomes with the monoyloid chromosome iiumher are 

 combined into a diploid chromosome complement, each kind of chromosome 

 then being present in duplicate. Every chromosome of this complement 

 divides equationally at every somatic mitosis in the development of the result- 

 ing new individual, so that every nucleus in this individual contains a 

 descendant of every chromosome originally present in the zygote. The 

 peculiar significance of these facts with respect to hereditj^ is evident 

 when it is borne in mind that the two genomes usually come from two 

 parent individuals, that they may exert somewhat different influences 

 upon the characters developed, and that they are to be reshuffled by 

 meiosis to form new genomes before the next sexual generation is 

 produced. 



Cleavage. — The rapid succession of meiosis, syngamy, and cleavage 

 in many animals has required frequent mention of cleavage in the fore- 

 going descriptions. This process, also called segmentation , is one of much 

 cytological interest and a few features of its early stages will be sketched 

 briefly. The subsequent course of embryogeny lies beyond the scope of 

 this book. It is the early cleavage divisions that furnish such excellent 

 material for studies on mitosis and especially of cytokinesis in animals 

 (page 69). 



The animal egg commonly shows a polarity of such a nature that one 

 region, the "animal pole," is physiologically more active than the 

 diametrically opposite region, the "vegetal pole." Also, eggs of different 

 animals differ greatly in the amount and location of their yolk material. 

 These features, to mention only two, exert a strong influence upon the 

 determination of the various cleavage patterns encountered in different 

 classes of animals. 



The geometrically most regular cleavage pattern is found in eggs 

 having their yolk uniformly distributed throughout the cell (homolecithal 

 eggs). Among echinoderms, for example (Fig. 94, A), the first cleavage 

 division is meridional (through the two poles), the second meridional 

 at right angles to the first, the third equatorial, and the several following 

 divisions in such planes as to result in a spherical mass of cells (blastomeres) 

 of uniform size. As development proceeds this sphere becomes a hollow 

 hlastula, and this in turn is converted into a gastrula by an invagination 

 which begins at the vegetal pole. 



The egg of the frog is somewhat telolecithal, i.e., its yolk tends to be 

 denser in the region of the vegetal pole than near the animal pole. The 

 first and second cleavage divisions occur as in the homolecithal egg, but 

 the third division is unequal, giving four small cells (micromeres) at the 

 animal pole and four larger ones (macromeres) at the vegetal pole (Fig. 



