CHEMODIFFERENTIATION 53 



then divides into a number of large cells, the teloblasts. The 

 latter remain at the surface of the germ, and, by budding off 

 small cells toward the anterior end of the embryo, form a left 

 and a right ectodermal germ-hand (Fig. 17d). The second 

 somatoblast also divides into two. Its two daughter-cells, the 

 primary mesoderm cells, move into the deeper layers where, 

 again by budding off cells unilaterally, they give rise to a left 

 and a right mesodermal gerhi-hand (Fig. 17c) lying under the 

 ectodermal germ-bands of the same side. During further devel- 

 opment, left and right germ-bands gradually unite, the process 

 beginning at the anterior end. Together they form the primor- 

 dium of the future embryo. The nervous system and the circular 

 musculature will arise from the ectodermal germ-bands, 

 whereas the longitudinal musculature, the segmental excretory 

 organs, etc., will be produced by the mesodermal germ-bands. 

 Skin and gut will originate from the remaining cells which 

 contain no pole-plasm. 



Penners irradiated the eggs with a very narrow pencil of 

 ultraviolet light. In this way he was able to kill certain cleavage 

 cells, thereby excluding them from further development. He 

 proved that the pole-plasm was indispensable for the formation 

 of the embryo. If the cells containing the pole-plasm had been 

 killed, the remainder of the e^g was unable to produce an 

 embryo. If, on the other hand, a number of other cells, together 

 constituting a large part of the volume, were eliminated, then 

 an abnormally small, but harmoniously built embryo would be 

 produced, so long as the somatoblasts remained intact. Elimina- 

 tion of the first somatoblast led to absence of the ectodermal 

 germ-bands, whereas the mesodermal germ-bands would be 

 present. The killing of the second somatoblast produced the 

 opposite result. 



Application of heat causes abnormalities in the cleavage of 

 these eggs. In this case, the pole-plasm is often not restricted 

 to one of the first two cleavage cells only, but distributed evenly 

 over both of them (Fig. 19a). In further development, each of 

 the blastomeres will form a first and a second somatoblast, 

 containing pole-plasm (Fig. 19b). Two embryo primordia will 

 develop in such a germ; together they form a double monster 



