ON THE DEVELOPMENT OE NEBALIA. 419 



main he was in agreement with Reichenbach for he considered 

 the proliferation in B ranch ipus to be virtually the same 

 thing as the invagination in the Crayfish. Also, it is quite 

 clear that he looked upon the zone of growth in the adult 

 Neb alia as having originated in either a proliferation or an 

 invagination. 



Other Ectodermal Structures. — The stomod^um has 

 grown considerably, and the shape of the future stomach is 

 foreshadowed more definitely than it was in the last stage. 

 Its two lateral walls are now composed of deep columnar cells 

 arranged so as to form two curved pads which nearly meet in 

 the middle (fig. 61, st.). 



The labrum has grown larger, and in sections one can see 

 that jointing has begun in the antennae (figs. 57 — 59). 



In the region of the mandibles the thickening of the dorsal 

 ectoderm of the body wall mentioned in Stage E can again 

 be seen (fig. 61). This, I think, is precedent to the formation 

 of the dorsal part of the shell. 



The two lateral shell thickenings have greatly increased in 

 size, and fig. 64/. shows a fold beginning to form between 

 one thickening and the rest of the ectoderm in its region. 



Endoderm. — The yolk is now surrounded by endoderm 

 except in the most anterior part of the embryo. In the 

 thoracic region two latero-ventral outgrowths of the endoderm 

 (which here completely surrounds the yolk) can be seen 

 (figs. 63 and 64). These I take to be the beginnings of the 

 liver lobes. It is in tbis way that the hepatic lobes in My sis 

 first make their appearance (Nusbaum, 1887). 



Metschnikoff (1868) in describing this stage states that 

 the yolk which is surrounded by endoderm is a coherent fluid 

 mass, while that in the anterior region which is un surrounded 

 is broken up into cone-shaped lumps. My sections show a 

 similar difference between the surrounded and uusurrounded 

 yolk. Also, in the surrounded yolk I find no vitellophags, 

 while in the unsurrounded portion some of these cells are 

 still to be seen. This, I think, goes to prove that the 

 vitellophags do in some way soften the yolk, and that when 



