﻿126 



William S. Marshall and Paul H. Dernehl, 



Donacia a number of clear spots near the periphery; they appeared 

 flrst on the ventral surface: lie followed Weismann (59) in believing 

 their origin to be a free cell formation. These nuclei later become 

 the nuclei of the blastoderrn cells which Surround the egg in a Single 

 layer. The blastoderrn of the ventral surface becomes thicker than 

 the rest, tliis thickening beginning at the niiddle of the ventral sur- 

 face and extending towards both poles but more marked at the 

 anterior. By Pecliculus a similar blastoderrn formation was observed. 



In 1869 Ganin (16) found that the blastoderrn cells in Hymeno- 

 ptera arise flrst at the posterior pole and then spread, the cells at 

 the poles being the largest, dorsally the smallest. In another paper 

 published the same year on the development of Platygaster (Ganin, 17), 

 he endeavoured to compare the entire yolk to the Keimhautblastem 

 of other insect eggs. In a third paper he studied the development 

 of the Diptera (Ganin 15); he showed the appearance of nuclei in 

 a peripheral layer and distinguished a convex ventral surface. The 

 same year Brandt (4) found that in the formation of the blastoderrn 

 by Odonata the cells appear at the periphery in groups and that 

 later the blastoderrn becomes thickened on one part of the surface. 



In 1870 Grimm (20) studied the development of Chironomus, 

 finding a layer, the Keimhautblastem, covering the egg; this was in 

 some parts thicker than in others. A Single nucleus gaye rise, by 

 division, to others, and these moved to the periphery of the egg 

 where a blastoderrn was formed. The blastoderrn cells later divide 

 transversely, resulting in the formation of two layers. Grimm (21) 

 also described the same process of blastoderrn formation in Docophorus. 



Bobretzky (3), in 1870, studied the development of two Lepido- 

 ptera by sectioning the egg, and he showed conclusively that the 

 nuclei of the blastoderrn cells come from those nuclei which are 

 earlier within the egg. These »cells« within the egg are amoeboid 

 and connected by fine processes. The blastoderrn cells are at first 

 large and far apart, becoming smaller and packing closely together. 

 Some »cells« remain in the yolk. He held that the amoeboid cells 

 came from the flrst segmentation nucleus. Previous to this paper by 

 Bobretzky, appeared two important works on insect embryology, 

 the one by Kowalevsky (31), the other by Bütschli (9). 



The Formation of the Blastoderrn. 



The general form of the egg of Polistes is ovoid with one pole 

 wider than the other. The widest part of the egg is towards the 



