EARLY DEVELOPMENT 37 



3. In the centipede Scolopendra, in the Thysanura (Lepifima), some 

 Homoptera (coccids, certain aphids), Heteroptera (Rhodnius prolixus), 

 and some Lepidoptera (Endromis, Ephestia, Solenohia) the germ cells are 

 recognizable shortly after the germ band is differentiated but before the 

 formation of the inner layer. 



4. In some Orthoptera {Gryllus, Periplaneta, Blattella, Melanoplus), 

 some Coleoptera (the tenebrionids Tenehrio and Tribolium), and some 

 Heteroptera (Pyrrhocoris) the germ cells are apparent during gastrulation 

 but before the formation of the coelomic sacs. 



5. In the Orthoptera {Xiphidium ensiferuni, Locusta migratoria) , 

 Lepidoptera {Bomhyx mori), Coleoptera (Hydrophilus piceus), and 

 Hymenoptera (Apis mellifica) the germ cells are first apparent after the 

 coelomic sacs are formed. Roonwal (1937) questions the correctness of 

 the position of Melanoplus differentialis in Section 4 above where it is 

 placed by Nelsen (1934) on the ground that a fundamental difference in 

 the origin of the germ cells is hardly to be expected in such closely related 

 species as L. migratoria and M. differentialis and that cardioblasts in the 

 latter species were probably mistaken for germ cells by the describer. 



The origin of the germ cells has not been described in the Odonata, 

 Ephemeridae, Neuroptera, and some smaller orders. 



Nelsen (1932) has attempted to correlate the various germ-cell origins 

 in insects b}^ making the following assumptions: (1) The germ plasm, 

 embryologically, is composed of two components, viz., the nucleus con- 

 taining its complete genie constitution and a rudimentary organization 

 contained in the cytoplasm of the egg ; (2) the cytoplasmic component is 

 similar to other organ-forming rudiments; (3) the entire developmental 

 history of this rudiment from the egg to the gamete is considered epi- 

 genetic; and, finally, (4) the germ-plasm rudiment located in the germ 

 cytoplasm may pass directly from the egg with the nuclear component in 

 the form of "pole cells," or it may take a more circuitous route together 

 with other organ-forming rudiments until it is ultimately segregated and 

 differentiated in the form of recognizable germ cells. 



Nelsen's view concerning the origin and segregation of the germ cells 

 does not sustain the idea of germ-plasm continuity and the "germ-cell 

 track" in a Weismannian sense, nor does it concur in the belief that germ 

 cells may be derived from a somatic origin. 



Seidel (1924) has endeavored to show that with the following insects 

 in the order given, Japyx, Lepisma, Pyrrhocoris, Orthoptera, Hydrophilus, 

 Calligrapha, Aphidae, Isotoma, and Diptera there is a continuous series 

 from the nondeterminative to the determinative type and that the 

 variation in the time of the appearance of the germ cells is directly corre- 

 lated with the general character of the development. With the non- 

 determinative type the differentiation of the germ cells occurs after the 



