338 



Embryogenesis: Progressive Differentiation 



merit may be found. For instance, in the 

 dragonfly Platycnemis, twin embryos of har- 

 monious propoi'tions can be experimentally 

 produced by injuring of the presumptive 

 germ band in early cleavage stages. In this 

 form, the embryonic material maintains its 

 regulative capacity until some time after the 

 blastoderm is formed. In Diptera, on the 

 other hand, the determination of the pre- 

 sumptive embryonic parts is already ac- 

 complished at the time of fertilization, for 

 when egg parts at this early stage are re- 

 moved, the remaining parts are unable to 

 develop beyond their prospective significance 

 (Reith, '25; Pauli, '27). An intermediate 

 position between these two extremes is taken 

 by the honeybee (Schnetter, '34) or the pea 

 weevil Sitonia (Reith, '35); in these the 

 determination of the embryo is completed 

 at the early blastoderm stage. The regula- 

 tive capacity of the embryonic material is 

 gradually restricted and finally lost as de- 

 velopment proceeds. In Platycnemis, where 

 in early development the various regions of 

 the presumptive embryo can still replace 

 each other, these regions become more and 

 more committed to special tasks with con- 

 tinued development. Finally the entire em- 

 bryonic zone reaches the mosaic stage, in 

 which each part is irrevocably determined 

 and destined to a specific end. This state of 

 affairs prevails not only in the determination 

 of the early embryonic material but also in 

 the determination of parts within organ 

 fields of larval or imaginal character, and 

 extends even to the processes of regenera- 

 tion: it is a general principle and well 

 founded indeed. 



Thus the principle of progressive deter- 

 mination applies to insects as well as to 

 other forms. Is the difference between regu- 

 lative and mosaic insect development to be 

 explained by this principle? If, as we usually 

 assxmie, it is the early or late determination 

 of embryonic parts for their final destiny 

 that distinguishes the regulative from the 

 mosaic type, then this principle does not ap- 

 ply. For eggs exhibiting mosaic characteris- 

 tics in early stages of development can in 

 the course of their development acquire the 

 capacity to regvilate (Ewest, '37). The differ- 

 ences between the determinate and indeter- 

 minate egg type are not based on alterations 

 of events in time, but are found in the spe- 

 cialized architecture of the different insect 

 eggs. Specializations in the configuration of 

 the internal egg structure which restrict 

 or enhance the freedom of movement of the 

 embryonic material in the dynamic egg 



system are the factors that decide and dis- 

 tinguish the determinate from the inde- 

 terminate type (Seidel, Bock, and Krause, 

 '40). 



EMBRYONIC INDUCTION BETWEEN GERM 

 LAYERS 



Among the causal factors of embryonic 

 determination, we must include the processes 

 of embryonic induction. The experiments 

 supplying this important information for 

 insects were carried out on the eggs of the 

 neuropteron Chrysopa perla (Bock, '39). The 

 decisive facts are briefly as follows: When 

 at an early stage of germ layer formation 

 ectoderm is removed at one side of the seg- 

 ment, the mesoderm spreads normally be- 

 low the ectoderm-free regions, but fails to 

 form an epithelium and soon degenerates. 

 Thus the epithelization of this layer is de- 

 pendent on the presence of ectoderm. The 

 question whether the ectoderm also decides 

 the further differentiation of the underlying 

 mesoderm was determined by other experi- 

 ments. Within one half of a segment large 

 parts of ectoderm located near the median 

 line or more lateral to it were removed. The 

 mesoderm below the median as well as the 

 lateral remains of ectoderm formed coelom- 

 epithelium of diminished proportions, corre- 

 sponding in size to the reduced ectoderm 

 portions above. The epithelium differenti- 

 ated in due course into intestinal muscles 

 and cardioblasts below the lateral ecto- 

 derm, while these differentiations were ab- 

 sent below the median ectodermal region. 

 In other words, the mesoderm always differ- 

 entiated according to position, regardless of 

 its prospective significance. From these re- 

 sults and from other experiments showing 

 that the mesoderm was isopotent for the 

 structures in question, it was concluded that 

 the various ectodermal regions determine the 

 t5rpe of differentiation of the mesoderm be- 

 low them. The lateral ectoderm regions hence 

 contain factors necessary for the formation 

 of intestinal muscles and cardioblasts, which 

 factors w^ere absent in the median ectodermal 

 portions. Thus the capacity of the ecto- 

 derm to imprint its specific demands on the 

 underlying mesodermal substratum must be 

 considered as a phenomenon of real em- 

 bryonic induction. In the light of these facts, 

 it is interesting to venture a comparison of 

 the types of organizing events between am- 

 phibians and insects. The decisive factors 

 for the organization of the embryo are lo- 

 cated in a definite germ layer in amphibians 



