Chapter 3< 



Embryonio development and Larvai Stages. 



1. Embryonic deyelopment (To Nauplius Stage). 



P eltogaster curvatus (Kossmami). 



(Piate 4.) 



The whole of the embryonic development to the Nauplius stage is passed through in 

 the matemal mantle-cavity. 



We have already seen, p. 29, that the first segmentation division takes place at the peri- 

 phery of the egg, and that at first the two daughter nuclei resulting from the division are split up 

 into a number of chromomeres. These separate bodies rapidly fuse to constitute an ordinary 

 nucleus; the furrow spreads till it cleaves the egg into two equal parts, and so we obtain 

 the two-cell stage figured (Piate 4 fig. 1). In a section through the upper hemisphere the 

 two nuclei are seen reconstituted (fig. 2). These two nuclei now divide simultaneously, and 

 a furrow, at right angles to the first, cleaves each celi into two. Thus is reached the four- 

 cell stage (Piate 4 fig. 3), a section through the upper hemisphere being shown in Piate 4 

 fig. 4. So far the cleavage has been regular, total and equal, but at this point irregular- 

 ities occur. 



I may mention at this point the researches of P. Abric on the cleavage of the egg 

 of Sacculina cardili. This had already been described by E. van Beneden (1), but Abric (3) 

 undertakes to correct this author in some points. Thus he finds that in the first two divisions 

 the daughter nuclei differ in size at first, a micromere being as it were budded off from a 

 macromere; the two products are however quickly equalised in size. The author also calls 

 attention to the frequent irregularities in the order of segmentation of the first few cells. 

 With regard to the inequality in size of the products of the first swo segmentation divisions, 

 I am unable to confimi Abric's observation for Sacculma, and I do not think that it is to be 

 observed in Peltogaster. 



