68 



GENERAL EMBRYOLOGY 



dimensions, and proportions (Fig. 33) (Montgomery). More- 

 over these differences are constant from one cell generation to 

 the next, so that similar chromosomes may be identified in 

 successive mitoses. The form is somewhat more variable than 

 the volume, which is remarkably uniform. It should be said 

 that the size of a given chromosome is not fixed throughout the 

 entire cell history, for at certain periods, particularly in the 



FIG. 33. Various chromosome groups illustrating variation in size and form, 

 and coupling of chromosomes. A,B, from Sutton, C, after Wilson, D, after Agar. 

 A,B. Spermatogonia of the grasshopper, Brachystola magna. C. Spermato- 

 gonium of the squash-bug, Anasa tristis. D. Spermatogonium of the lung-fish, 

 Lepidosiren. 



germinal tissues, the chromosomes may be many times larger 

 than at other periods (Fig. 34). But at corresponding cell ages 

 the corresponding chromosomes are practically equal in volume, 

 and in somatic cells such volume changes of single chromosomes 

 are relatively infrequent. 



One most significant and very important fact in this con- 

 nection is that in the somatic cell the chromosomes are pres- 

 ent in couples of similar elements; there are two of each size or 

 form (Montgomery) (Figs. 33, 72, A; 142, A). The exceptions 



