EXPERIMENTAL EMBRYOLOGY 149 



egg parts as determined by the regulative power or, conversely, the degree 

 of predetermination of the egg parts. These studies have shown that 

 Seidel's series is fundamentally accurate both for visible differentiation 

 and for the degree of predetermination of the various egg parts. From 

 both viewpoints it is a matter of time (time or stage of visible differentia- 

 tion and time of determination), indeterminate eggs becoming more and 

 more determinate as development proceeds, determinate eggs being fully 

 determined by the time of fertilization. Schnetter gives the following 

 revised scheme: 



Indeterminate Incom pletely Determi nate Determinate 



Odonata-Hemiptera-Orthoptera-Coleoptera-Hymenoptera-Lepidoptera Diptera 



Whether or not Schnetter is justified in listing the foregoing sequence 

 seems open to question. In the first place no experimental data are 

 available for either the Hemiptera or the Lepidoptera, and these two 

 orders are placed in the series wholly on a basis of the time of visible 

 differentiation of the various parts. A more serious criticism lies in the 

 impUcation of homogeneity within single orders. Experimentally very 

 few species have been studied, these being representative of the orders 

 Odonata, Orthoptera, Hymenoptera, Neuroptera, Coleoptera, and 

 Diptera. There is good reason to expect considerable variation within 

 the order Hymenoptera, polyembryonic forms seeming to be highly 

 "indeterminate." Perhaps similar variation will be found within other 

 orders. 



Indeterminate eggs have a certain amount of determination at the 

 time of fertilization. Bilaterally symmetrical ones have the polarity 

 and dorsal and ventral sides irrevocably determined. In the absence of 

 experimental data it is not possible to evaluate the variation in position 

 of the embryo of the radially symmetrical egg of the bug Pyrrhocoris; 

 though the embryonic axis varies through 90 degrees, it is not possible to 

 say whether its position is determined before, during, or after fertiliza- 

 tion. Except for the fundamental axes, indeterminate eggs possess great 

 regulative powers, as shown by dwarf embryos and duplications. Experi- 

 mentally produced twins and duplicated parts show that there is in the 

 early blastoderm a specific disposition of materials for the formation 

 of the germ band but that when these materials are separated, each 

 part tends to form a whole structure rather than only an isolated part. 

 This power becomes more limited as development proceeds, but studies 

 on regeneration show that the power of regulation is not entirely lost 

 until the adult stage. 



In Platycnemis Seidel (1928, 1929a) reports that shortening the 

 longitudinal axis by constricting the egg results in dwarf but otherwise 



