ON THE DEVELOPMENT OF NEBALIA. 403 



continuity with those of the second on either side, and these 

 two pairs of ganglia lie in front of the mouth, Avhile those of 

 the anteunary and mandibular segments lie behind it. 



In one specimen a section through the first antenna shows 

 the ganglion near its base to consist of two kinds of cells, 

 as it does in Stage D. This is only worthy of notice as an 

 instance of a difference in internal development between 

 embryos which have the same external features. 



Other Ectodermal Structures. — I find in this stage 

 the rudiment of a labrum (fig. 24), and also a well-marked 

 stomodteum, which runs a little forward from the mouth (fig. 

 26), but, though I have looked through several series of both 

 transverse and longitudinal sections, I can find here no 

 beginnings of either anus or proctodseura. 



Endoderm. — The solid band of cells resulting from the 

 invagination at the blastopore now spreads out laterally, i. e. 

 its cells multiply laterally, so as to gradually enclose the 

 yolk. The enclosure seems to take place in this way. Start- 

 ing from the anterior end of the caudal thickening, just 

 behind the depression, between it and the thorax, the cells 

 grow backwards and dorsalwards, so that the first part of the 

 yolk to be enclosed is that lying in the papilla (fig. 26). 

 When this is done, i. e. when the yolk within the papilla is 

 enclosed, the cells seem to increase more rapidly on the 

 dorsal than on the ventral surface. A transverse section in 

 front of the papilla through the farthest advanced of the 

 embryos with nauplius appendages shows endoderm lying on 

 the dorsal surface of the yolk, but none on the ventral 

 surface. Similarly, a longitudinal section through an embryo 

 belonging to the next stage shows endoderm stretching much 

 farther forward on the dorsal side than on the ventral (see 

 Diagram 2, p. 408). The endoderm cells in this stage begin 

 to assume a columnar shape. 



Mesoderm. — The mesoderm can be traced lying between 

 the ectoderm and the yolk in chains of three or four cells on 

 the ventral side, from the abdominal papilla to the optic 

 invaginations. It is most abundant in the region just in 



