508 A. M. OAlMi SAUNDKK8 ANJ) MARGARET POOLE. 



Blochmann, who assert that it closes and then reopens to 

 form the mouth. The stomodaeum does not at this stage 

 communicate Avith the space between the endomeres ; it is a 

 blind sac lined by ectodermal stomatoblasts and cesopiiago- 

 blasts. The former, according to Carazzi, are dei-ived from 

 Sa^ and 3/^', and the latter from 3a- and 'Sir ; togetlier they 

 number between twenty-five and thirty. On the dorsal 

 surface of tlie embryo and posterior is the shell-gland. 

 It consists of a deep and narrow invagination, formed by a 

 large number of cells, which are slightly differentiated from 

 the adjacent ectoderm by their more elongated shape and 

 rounder nuclei. 



On each side of the embryo, just ventral to a line joining 

 the anus and the mouth, a small ectodermal invagination, ot., is 

 seen to be in process of formation. These are the pair of oto- 

 cysts. Fig. 3 shows the same structures at a slightly later 

 stage. A little anterior to the anal cells on the right side are 

 four large ectoderm cells identified by Carazzi as 3c'*", Sc^^^^, 

 Sc^~^^ and 3c*~*~. These cells are at this time clearly in the 

 ectodermal layer, but they soon sink below the surface and 

 give rise to the secondary kidney. Their nuclei are of great 

 size, and generally each contains one prominent darkly 

 staining plasmosome. 



The greater part of the interior of the embryo is occupied 

 by the two large endomeres A and B (not lettered in the 

 plate). They diverge somewhat from one another, and thus 

 enclose between them an irregularly triangular segmentation- 

 cavity (marked .v/. in tigs. 2 and 3). The broad end abuts upon 

 the shell-gland posteriorly, while the narrow end reaches to the 

 bottom of the still blindly-ending stomodieum. The nuclei of 

 the endomeres are large and oval in shape, lying to the inner 

 side of the cells near the segmentation cavity. The cytoplasm 

 is heavily laden with large yolk-granules, and some of the 

 yolk is often found in the nuclei also, causing the latter to 

 stain very deeply with plasma stains. A large vacuole is 

 generally present in each of the endomeres, which is very con- 

 spicuous in the living embryo, and persists for a long time. . 



