EXPERIMENTAL EMBRYOLOGY 161 



(1935) for Platycnemis. This is well shown in Fig. 61, which gives the 

 blastoderm plan from all three views, and in Fig. 62, which shows the 

 changes undergone during the formation of the embryo. The first phase 

 of shortening of the embryo occurs simultaneously with the onset of 

 action of the differentiation center (Fig. 62a,6). During this time the 

 presumptive head and abdominal regions contract while the thoracic 

 region increases in length as a result of two movement tendencies, one 

 dramng the materials toward the region of the differentiation center 

 (between the gnathal and thoracic anlagen), the other drawing the entire 

 presumptive embryo toward the posterior end of the egg. The two 

 tendencies coincide in front of the differentiation center but are opposed 

 posterior to it. In the second phase of shortening (Fig. 626,c) the head 

 and gnathal anlagen expand while the thorax and abdomen shorten. 

 During this phase the action of the differentiation center is no longer 

 apparent. The entire embryo continues to move posteriorly and soon 

 invaginates into the yolk. During and immediately following invagina- 

 tion the parts of the embryo elongate and asssume larval proportions. 



From Seidel's detailed discussion of irradiation defects as marks in 

 making these maps we note the following: (1) The study is hindered by 

 regulation processes. (2) Raising the temperature accelerates the dif- 

 ferentiation process more than the healing process and so aids the forma- 

 tion of defects. (3) For certain organs the experiments show a "defect 

 correlation" rather than a "developmental correlation." For instance 

 there is a high correlation of eye and thoracic defects which Seidel sug- 

 gests may be due to a primary defect changing yolk contraction and 

 thereby altering the molding of the embryonic anlage. (4) Defect experi- 

 ments do not show the true morphological course of development owing 

 to the indirectness of physiological investigation; more truly they indi- 

 cate a plan of the various factor regions. 



Schnetter (19346) gives a partial segmental map of the 12- and 24-hour 

 blastoderm of the honeybee egg (Fig. 60) showing a shift in the prospec- 

 tive significance of the parts of the blastoderm between these two 

 stages. Incidentally he says that after the formation of the germ 

 band, the differentiation center no longer belongs to the structure of the 

 whole egg but only to the embryo. With the shift of the presumptive 

 embryonic parts the middle of the differentiation center shifts from 

 division 24 of the egg to division 28. 



For the determinate (mosaic) egg of the Diptera, Reith (1925) and 

 Pauli (1927) show that the parts of the embryo originate as presumptive 

 anlagen at the same points where they later make their appearance and 

 that little or no regulation occurs. Sonnenblick (1934) and Howland 

 and Child (1935) report that normal larvae and adults may develop 

 from punctured Drosophila eggs from which a portion of the contents 

 has been extruded. Owing to doubt as to the exact nature of this 



