ON TFIE DEVELOPMENT OF NEBALIA. 393 



The Development of the Embryo. 



The Formation of the Blastoderm. 



Metschuikoff describes a discoidal segmentation ; but a 

 series of sections made through a stage which is earlier than 

 that shown in Plate 16^ tig. a, leads one to suppose that we 

 have in Neb alia not an instance of discoidal segmentation 

 proper, but rather a case of Korschelt and Heider's (1893), 

 type Illb, in which the increase of the blastoderm is aided 

 by the accession of new elements from the inside of the egg. 

 It seems that the blastoderm, certainly at first, increases both 

 by division of the cells already on the ventral surface and also 

 by the reception of additional cells which come from within 

 the egg itself (Plate 16, fig. 1). According to Butschinsky 

 (1897) the first two divisions take place in the protoplasm 

 while it is still lying in the centre of the egg within the yolk. 

 The four cells resulting from these divisions travel to the 

 ventral pole of the egg and there divide, thus forming a cap 

 of eight cells. This cap, by subsequent divisions of its cells, 

 gradually surrounds the yolk. He also describes a few cells 

 which are smaller than the other cells of the blastoderm. 

 These smaller cells have escaped my observation. 



Stage A. (Plate 16, fig. a.) 



Figure a shows an egg in which the yolk is still incom- 

 pletely covered by the blastoderm. Sections through it 

 contain none of the large cells within the yolk shown in 

 fig. 1. Therefore it may be supposed that only in a very 

 early stage is the blastoderm increased by the reception of 

 additional cells coming from within the yolk. 



The blastoderm cells are more or less oval in shape and 

 their protoplasm is granular. The yolk is broken up into 



