400 MARaAEET EOBINSON. 



between the development of My sis and that of Nebalia in 

 these early stages which show the origin of the endoderm 

 from an invagination which results in the formation of a 

 solid band of cells. 



Stage C (fig. c). Embryo with Nauplius 

 Appendages. 



External view. — In an external view of the ventral sur- 

 face of the embryo at this stage one can distinguish : 



(1) The eye thickenings. 



(2) The first and second antennae. 



(3) The mandibles. 



(4) The abdominal papilla which is bent forward. 



(5) The mouth which is here a crescent-shaped depression 

 lying between the first and second antennae. 



Internal structure. — As is usual among the Crustacea, 

 embryos of approximately the same external appearance vary 

 not only in size but also in internal development. I have 

 therefore made drawings from three series of transverse 

 sections taken from three embryos, each of which has the 

 nauplius appendages. It is chiefly in the ectoderm of this 

 stage that differences occur. 



All three series show the optic thickenings in the anterior 

 region, though in the very front these thickenings are not 

 much pronounced, and indeed show little or no advance in 

 size on those of the preceding stage (fig. 16). All three too 

 show at a little distance behind the most anterior part of the 

 optic thickenings on either side an invagination of ectoderm 

 cells (op. in. fig. 17). This very closely resembles the optic 

 invagination described by Reichenbach (1886) in the cray- 

 fish embryo with nauplius appendages. 



In each of the three series, almost immediately behind the 

 optic invagination on either side, there are a few ectoderm 

 cells which are much larger than the rest, almost double the 

 size, with large nuclei (tig. 18 y.i). These I take to be the 



