EMBRYOLOGY. 243 



larva is evident from a comparison with the figures of earUer stages. In the 

 oldest larva the increase in the size of the body is not especially marked, and 

 yet the test does not compose more than half of the external surface. Measure- 

 ments, somewhat roughly made, indicate that a shrinkage of approximately 

 two fifths of the original superficial extent of the test has taken place. To what 

 extent this progresses is not known; nor is it known what means are employed 

 to increase the area of the remaining ectoderm. In a few cases cells of the trunk 

 ectoderm have been seen, containing karyokinetic spindles thus indicating one 

 source of increase. On the other hand there are certain large, yolk containing 

 cells in the anterior end of the body of the oldest larva (Plate 14, fig. 4) that do 

 not appear to belong to the mid-gut. From their position it is altogether pos- 

 sible that products of these are added to the external layer as the free border 

 of the test advances toward the apical pole. There is no indication other than 

 this of an ectodermic layer beneath the test, and the indication is that at the 

 time of its dehiscence the test is a comparatively insignificant organ. 



Nervous System: — In the earliest recognizable stage the cerebral ganglia 

 appear (Plate 14, fig. 6) as a set of cells bordering a depression in the test. It 

 is evident that originally one or more cells, indistinguishable in sections from 

 those of the test, underwent cleavages in which the plane of division cut the 

 surface of the body at right angles. The resulting elements migrated some 

 distance into the interior of the embryo, and at a later time other cells were 

 cut off from these parent cells that remained in contact with the depression. 

 By the successive divisions of daughter and parent cells a large accumulation is 

 produced, extending from the exterior to the neighborhood of the stomodaeum. 

 Within a comparatively short time wing-like prolongations are developed wluch 

 encircle the stomodaeum and still later these are continuous with a rod-like 

 mass resting against the ventral ectoderm and extending to the posterior end 

 of the body (Plate 4, fig. 3, 4). Unfortunately all of the sections of the later 

 stages are longitudinal, and it is not possible to determine if this ventral band 

 is double, as it ultimately must become if my beUef is correct that it forms the 

 ventral cords. 



While the evidence goes to show that the cerebral gangUa arise at one point 

 in the outer layer of cells, later stages indicate that the depression undergoes a 

 considerable lateral expansion, and in one case the two accumulations, destined 

 to form the halves of the brain, become almost if not completely separated 

 from each other, there being two external pits in contact with the surface. In 

 another example the nerve masses are at opposite ends of a transverse groove 



