314 GENERAL EMBRYOLOGY 



relations, whatever they are, are constant within a given 

 species. 



One further general condition of the idiochromosomes must 

 be noted. In all of the instances mentioned above the sper- 

 matozoa are the dimorphic gametes, i.e., the males are "diga- 

 metic" (Wilson's term). In a few species, however, it is the 

 female that is digametic, and while the spermatozoa are all 

 alike, the ova are of two classes with respect to certain modified 

 chromosomes which may properly be regarded as homologous 

 with the X- and Y-elements of the spermatozoa. Thus in two 

 of the sea-urchins, Strongylocentrolus, and Echinus, Baltzer 



FIG. 147. Chromosomes in the sea-urchin, Strongylocentrotus lividus. After 

 Baltzer. X 2610. A. First cleavage spindle (reconstructed from two draw- 

 ings). No small hooks present. B. First cleavage spindle with small hooks. 

 The modified chromosomes, both long and short hooks, are shown in solid black. 



finds the chromosome groups of the spermatids all alike, with 

 eighteen components uniformly differentiated; the nuclei of 

 some of the ova, however, are characterized by the presence 

 of a single modified component which is absent from the 

 remainder (Fig. 147). This obviously corresponds with the 

 X-element of the dimorphic sperm. There is also very good 

 reason for believing the female digametic in some of the 

 Lepidoptera; this is of considerable interest in view of the fact 

 that repeated investigation has thus far failed to disclose 

 idiochromosomes in the spermatocytes of these Insects. 



