194 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
apparent that its development took place wholly in the free-swimming 
stages. A transverse section through the antennule of a newly hatched 
larva (Plate 4, Fig. 19) shows no sign of invagination in the region 
where the sac is to appear. But certain elongated nuclei, evidently 
those of modified hypodermal cells, are found grouped, two or three 
layers deep, beneath the dorso-lateral wall of the appendage (Fig. 19, 
cl. ma.). These elongated nuclei, viewed from the dorsal surface of the 
appendage, are seen to be roughly arranged in a semi-circle, like the rows 
of otocyst hairs in Figure 26 (Plate 5), and when traced through later 
stages, the position they occupy is found to be directly beneath the ridge 
where the sensory hairs later appear (Plate 5, Fig. 24, set. of.). They 
are evidently, therefore, the nuclei of the matrix cells which build up 
by secretion the chitinous walls of the sensory hairs. These cells, like 
those which take part in hair formation after ecdysis, originate from the 
-chitinogenous hypodermal cells by simply becoming elongated and sink- 
ing beneath them. A similar arrangement of matrix cells was found in 
the developing otocyst of Mysis by Bethe (95*). Numerous spherical 
nuclei, which stain in a manner characteristic of nerve cells, are present 
just below the matrix cells (Fig. 19, n’bl.). . If traced back to the gan- 
glionic masses of the brain, they are found to be continuous with the 
nerve cells of the latter, and probably originate from them. 
b. Second Larval Stage. 
(Second to fifth pair of abdominal appendages present.) 
In this larva the first evidence of invagination is seen on the dorsal 
side at the base of the antennule (Plate 4, Fig. 20). The nuclei of the 
matrix cells are now larger, and very conspicuous at the lateral side of 
the transverse section, the region where the rows of hairs will later 
appear. Figure 22 (Plate 4) shows the anterior and posterior limits of 
the invagination and the fundament of the sensory ridge, marked by a 
fold in the hypodermis and chitin at el. ma. The matrix cells just 
posterior to this fold, whose processes are directed toward it, are those 
which are to form the transverse portion of the hair rows. As in the 
first stage, nuclei of nerve cells lie immediately beneath the matrix cells, 
but the cytoplasm about them shows as yet no definite boundaries or 
outlines, nor are there any signs of nerve fibres connected with them. 
e. Third Larval Stage. 
(Chele relatively larger, uropods present.) 
In this stage (Plates 4, 5, Figs. 21, 23) invagination has proceeded 
