140 



GENERAL EMBRYOLOGY 



Arthropods. In the great majority of animals the reduction 

 divisions proceed without the formation of actual tetrads in 

 their typical form, and when the bivalent chromosomes appear 

 in the nuclei of the primary spermatocytes, as the result of 

 synapsis, they have no unusual form. As they pass into the 

 equatorial plate, however, it is seen that the two longitudinal 

 halves, composing each bivalent chromosome, are united at 

 their ends (Figs. 72, E;75). When the anaphase begins each 

 chromosome-half is drawn out first from its middle, so that the 



FIG. 75. Maturation divisions in certain Insects, showing forms of chromo- 

 somes and their relation to tetrads. After de Sinety. X 1125. A,B. Two 

 stages in anaphase of primary spermatocyte division in Stenobothrus parallelus. 

 Rings opening into Vs, which diverge. C. Anaphase of spermatogonial division 

 in Orphania denticauda, showing differentiated chromosome, x. D, E. Prepara- 

 tion for first spermatocyte division in Orphania, showing "tetrads" in various 

 stages of formation from rings and crosses. 



whole original chromosome may appear as a ring or cross, or 

 some related figure. As the halves separate each may assume 

 a p- or > -form, the limbs of which may come to lie parallel as 

 the chromosome approaches the pole of the spindle, and thus 

 may appear to be double (Figs. 75, 20). On account of this 

 peculiar form assumed by the chromosomes in this division, 

 it is known as the heterotype division. And it is to be noted 

 that the separation of the halves of the bivalent chromosome 



