Insects 



339 



as well as in Chrysopa. In amphibians they 

 are found in the mesoderm, in Chrysopa in 

 the ectoderm. The inducing material of 

 amphibians enjoys a great freedom of move- 

 ment. The extent of these movements deter- 

 mines to a large degree the size and propor- 

 tions of the induced structures. In Chrysopa, 

 on the other hand, the inductor, the ecto- 

 derm, is a relatively rigid system, endowed 

 with a mosaic of inductive potency. The 

 mesoderm underlies this system and re- 

 sponds with specific differentiations to the 

 stimuli emanating from it. The movements 

 of the embryonic material, different as they 

 are in both classes of animals compared, 

 undoubtedly have the same basic signifi- 

 cance, namely the bringing together of de- 

 velopmental systems for the purpose of 

 interactions that provide the basis for the 

 organization of the embryo (Seidel, Bock, 

 and Krause, '40). 



CYTOPLASMIC DETERMINERS 



In the determinate egg type, the cortical 

 cytoplasm is usvially heavy and already in 

 the fertilized egg seems to consist of a mosaic 

 of differentials that determine the various 

 parts of the future embryo. When in the 

 beetles Leptinotarsa (Hegner, '11) or 

 Bruchus (Brauer and Taylor, '34) small 

 areas of the cortical cytoplasm are destroyed 

 before the cleavage nuclei have arrived 

 and without apparent injury to them, those 

 parts of the embryo are missing which would 

 have developed from the destroyed regions. 

 Since no degeneration of the cleavage nuclei 

 occurred, it follows that they must have 

 taken part in the development of some other 

 parts of the egg. The cortical cytoplasm 

 they happen to invade as the result of the 

 experimentally altered conditions must have 

 determined their fate. The fact that these 

 nuclei were able to respond to the cyto- 

 plasmic influences of regions normally for- 

 eign to them indicates their totipotency. 

 The totipotency of cleavage nuclei in early 

 stages of development, it might here be 

 added, was definitely proved in experiments 

 on other forms, notably Platycnemis, where 

 it persists at least until the seventh cleav- 

 age division (Seidel, '32). These experiments 

 show quite conclusively that the cortical 

 cytoplasm is a differentiated continuum in 

 which localized differentials exert specific 

 influences on the cleavage nuclei, leading 

 them towards special assignments. One has 

 to assume that the cytoplasmic regions pos- 

 sess their specific qualities only when in 



normal topographic relationship to the corti- 

 cal cytoplasmic layer as a whole. Their 

 influences must be regarded as of a general 

 directive nature in that they set up differ- 

 entials in the cleavage nuclei, thereby creat- 

 ing a definite pattern within the framework 

 of the blastoderm, which forms the basis for 

 the ensuing developmental events. If one 

 could excise the cytoplasm of the presump- 

 tive eye region and supply it with any 

 number of the totipotent nuclei, the isolated 

 bit of tissue would in all probability never 

 give rise to an eye or to any specialized 

 structure. 



Within the realm of cytoplasmic deter- 

 miners one has to include the pole-plasm. 

 The cortical cytoplasm at the posterior pole 

 of the egg is in certain insects distinguished 

 from the rest of this layer by the presence of 

 deeply staining granules. This region is 

 called the pole-plasm. The cleavage nuclei 

 which penetrate this region become known 

 as pole cells; they represent the germ-cell 

 primordia. When in the egg of the beetle 

 Leptinotarsa (Hegner, '11), the polar cyto- 

 plasm is removed before the cleavage nuclei 

 have entered the pole, germ cells are lack- 

 ing in the embryo. This shows that the polar 

 cytoplasm contains some factor essential for 

 the formation of the germ cells. The polar 

 region of the cortical cytoplasm is thus en- 

 dowed with a specific organ-forming prin- 

 ciple which takes effect at an early stage 

 in development and which determines the 

 cleavage nuclei towards their future des- 

 tiny. It has been assumed that the formative 

 role of the polar influences was such as to 

 predetermine rigidly the polar cells for 

 their fate. That this is not the case has 

 recently been demonstrated in Drosophila 

 (Poulson, '47). For some time it has been 

 known that not all the pole cells take part 

 in the formation of the gonad, but that 

 some of them go astray on their way to 

 their final location in the interior of the 

 embryo. These "lost" cells were formerly 

 supposed to degenerate, apparently because 

 of their failure to become germ cells. Now 

 Poulson has made the striking discovery that 

 these "lost" cells do not degenerate at all, 

 but that they can become incorporated into 

 the epithelium of the gut, of which they 

 actually become a part. This fact deserves 

 emphasis, for it demonstrates that the cyto- 

 plasmic pole factors do not decide finally 

 the ultimate fate of the pole cells. They 

 obviously endow the pole cells with the 

 potentialities necessary for the formation of 

 germ cells. But whether or not these or other 



