ON THE DEVELOPMENT OF NEBALIA. 409 



In tliis stage in the region of the first antenna there is a 

 decided thickening of the ectoderm on either side of the 

 body-wall dorsal to the antennas. These thickenings are 

 lateral, but more ventral than dorsal in position. I take 

 them to be the earliest traces of the shell valves (fig. 37, s.v.). 



Mesoderm. — There is no great advance in this layer upon 

 its condition in the embryo with nauplius appendages. We 

 find mesoderm as far forward as the optic lobes, though not 

 in the lobes. It is to be found in the appendages (i. e. in the 

 first and second antennse and mandibles (figs. 35 — 39). As 

 yet there is no sign of a split in the mesoderm except in the 

 second antenna (fig. 39) where there is the appearance of a 

 split which may, however, be accidental and due to reagents. 

 Vitellophags are still fairly abundant. 



Eudoderm. — This consists of large columnar cells with 

 large nuclei. It is still confined to the hinder part of the 

 body, extending on the dorsal side from the proctodeeum to 

 the region of the optic lobes. Ventrally it does not extend 

 so far forward. Only in the abdominal papilla does it 

 completely surround the yolk as is shown in the above 

 diagram (2). 



Stage E (fig. e). 



External View. — The chief new features to be noticed in 

 an external view of the embryo at this stage are the increase 

 in the length of the appendages, the appearance of the median 

 head flap, and the great increase in the size of the labrum. 



There is also a change in the position of the appendages. 

 They now more or less follow the outline of the body and are 

 directed backwards instead of standing out at an angle to 

 the body as they did in the last stage. The shape of the 

 body itself too has altered a little. It is now longer and 

 more oval than in Stage D. The posterior half is, however, 

 much narrower than the front end, and the papilla stretches 

 farther forward than it did in the last stage. 



Internal Structure. Ectoderm; the Nervous 



