ON THE DEVELOPMKNT OP NEBALIA. 415 



Stage F (fig. f). 



External View. — The embryo now lias burst its shell, 

 but is still enclosed in a cuticle. Metsclinikoff says of this 

 cuticle that ''it invests all the first five appendages (nauplius 

 appendages and maxillaB) closely following all their curves 

 and outgrowths. The rest of the appendages are covered 

 by an unbroken skin which forms a general sac over them 

 and the back.^' In looking at the embryo under a dissecting 

 microscope I have not succeeded in making out this cuticle, 

 much less its disposition, but in many of my sections I have 

 noticed pieces of it loosely enveloping the embryo. 



In breaking the shell the embryo has become uncurled. 

 It seems, in fact, to have sprang back with a rebound, so that 

 now there is a slight dorsal curvature of both head and tail. 

 It has now, besides the eyes, anteunge, mandibles, and maxillee, 

 seven, sometimes eight, thoracic appendages, sometimes also 

 the rudiments of three abdominal legs. The optic lobes are 

 now almost completely nipped off from the yolk. All the 

 thoracic appendages are bilobed so that if the embryo were 

 free-swimming one might call this the My sis stage. 



Internal Structure. Nervous System. — The nervous 

 cells are now leaving the surface, i. e. they now no longer 

 form part of the ectoderm of the body wall. This can be seen 

 in fig. 56 e.c, where there is shown a thin layer of ectoderm 

 lying outside the large dividing cells of the ganglion of the 

 antennule. 



Protocerebrum. — Concomitant with an increase in size 

 of the optic ganglia there is an increase in the number of 

 fibres which they contain. The nerve cells in these ganglia 

 have a very regular arrangement resembling that shown in 

 Nusbaum's figures of the brain of the Mysis embryo 

 (Nusbaum, 1887, fig. 80). 



The central mass of nervous tissue between the optic 

 ganglia has grown considerably. It is now very distinctly 

 bilobed, and, in section, can be seen to be clearly marked off 



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