224 



REPRODUCTION 



to perish with the death of the body. As developed by W'eismann 

 in the last quarter of the nineteenth century, this theory regarded 

 the germplasm as relatively stable and independent of changing 

 conditions in the somatoplasm. It was, therefore, in opposition 

 to the theory that the germ cells were a product of the body cells 



nc^ 



n.o. 



Fig. 112. — Early differentiation of germplasm. 



A, transverse section of late blastula stage jf a lamellibranch, Sphcerium atriatinum, 

 showing segmentation cavity (s. c.) and the two primordial germ cells (g. c.) from which all 

 the germ cells of the paired reproductive organs seem to originate; B, section of late ovarian 

 egg of an insect, Miastor americana, showing covering of follicle cells, the nurse cells (n. c), 

 the nucleus of the ovum (n. o.), and a differentiated area of cytoplasm (p) that is traceable 

 to the cytoplasm of the germ cells in the subsequent development {A, from an unpublished 

 drawing by F. H. Woods; B, after R. W. Hegner, Jour, of Morph., Vol. 25.) 



at the time of sexual maturity and subject to many of the influences 

 that affect the somatoplasm. The individual is a " chip of the 

 old block," but the " block " is the germplasm and not the parent's 

 somatoplasm. Although this idea was originally elaborated by 

 Weismann on theoretical grounds, it found concrete support in 

 the later discovery that the germ cells of many animals may be 

 identified at an early stage of the embryo before the reproductive 

 organs have become differentiated as such. Thus, in many 

 instances, it has been possible to trace the primordial germ cells of 

 the individual from the late cleavage stages (Fig. 112 A), and in 

 some instances to discover within the cytoplasm of the unfertilized 

 ovum (Fig. 112 B) substances that become localized in the germ 

 cells when these can be definitely recognized in the subsequent 

 development. More recently, however, it has been found in a num- 

 ber of cases that these early germ cells degenerate; and that the 



