118 THE DEVELOPMENT OP 



readily reduced to the normal alecithal type. The proto- 

 plasm segments, the yolk in most hexapods and some 

 Crustacea, does not, but this yolk in either group is to be 

 regarded as superficial rather than central, and the term 

 ectolecithal, though not necessary, is far preferable to 

 endo- or centrolecithal. The view I take of this segmenta- 

 tion is essentially the same as that of Bobretzky ('78) and 

 I fail to see the force of the objections raised to it by Bal- 

 four ('80, p. 98). The nuclei, and the surrounding proto- 

 plasm, are clearly to be regarded as cells, and that they do 

 move about with comparative freedom in the yolk is shown 

 in almost every hexapod and many crustacean eggs. The 

 segmentation of the yolk, like the yolk itself, is a second- 

 ary feature ; and the fact that it truly segments in Astactis, 

 Homarus and Eupagurus while in Crangon and Peneus it 

 does not, shows the slight importance of this point. In Eu- 

 pagurus several divisions of the nuclei and the surrounding 

 protoplasm take place before the appearance of the seg- 

 mentation planes which are to divide the yolk. A still 

 further postponement of their appearance would give us 

 the condition occurring in Crangon or in the Hexapods. 

 Balfour quotes his observations on the eggs of spiders 

 in support of his position, but Locy's observations on still 

 earlier stages of the same genus ( Agelena) are readily made 

 to support the view here adopted. The segmented hexapod 

 egg is not a syncytium ; the cells are completely divided 

 or nearly so and the intercellular spaces are occupied by 

 the yolk which is here certainly to be regarded as a second- 

 ary element in the egg. 



Having thus described the phenomena of segmentation in 

 the more common type of arthropod egg we may proceed 

 to the discussion of the gastrula, leaving until later the 

 meroblastic and holoblastic segmentation occurring in some 

 forms. 



