404 MAEGARET ROBINSON. 



front of the papilla. The mesoderm cells can be seen bein^^ 

 carried along with the ectoderm as it is being folded off to 

 form the nauplius appendages (figs. 20 — 22). 



Vitellophags. — These cells are now more numerous than 

 they were in the preceding stages. Wherever one of them 

 is seen the yolk round it has a granulated appearance. This 

 granulation may be a step onwards towards liquefaction. It 

 seems that the vitellophagSj while transforming the yolk, 

 undergo disintegration themselves. They certainly appear 

 to lose their own protoplasm, and their huge swollen nuclei 

 look as though they were ready to disintegrate (fig. 26, v.j).). 

 Fig. 26, v.}}., shows a number of vitellophags, some with a 

 little protoplasm, and others consisting of a nucleus only. 

 This disintegration certainly militates against any idea that 

 the vitellophags become blood corpuscles or take any part 

 in the development of the embryo other than that of render- 

 ing the yolk more easy of absorption by the protoplasm. 



Stage D (fig. d). 



External Features. — The chief advances noticeable in 

 an external view are the increase in the number of the 

 appendages, and the growth farther forward of the abdominal 

 papilla. 



The embryo now has, in addition to the nauplius appendages, 

 two pairs of maxilla) and rudiments of the first three pairs of 

 the thoracic appendages. The mouth has moved farther back, 

 so that now it is in the segment which bears the second ])air 

 of antenna). Behind the mouth there is a distinct median 

 ventral depression. In optical section the endoderm sur- 

 rounding the hinder two-thirds of the yolk can be seen dis- 

 tinctly. This stage, Clans notwithstanding, bears a likeness 

 to a Zoaea, and might certainly be called the stage with Zoaja 

 ap])endages. 



Internal Structure. Ectoderm ; the Nervous 

 System. — There is here a great advance in the complexity 

 of the nervous system. In each segment there is a well- 



