viii AETHEOPODA 189 



The cephalic lobes now project freely from the surface of the egg 

 as the eye-stalks, and the ectoderm cells covering them have become 

 several layers deep. These ectoderm cells then become arranged in 

 radially directed strings, each of which forms an ommatidium or eye 

 element ; the outermost cells giving rise by their secretion to the 

 corneal lens, those beneath them to the crystalline cone, whilst 

 from the innermost cells the retinula is derived (see Figs. 220-222). 



The coelomic cavities of the mesoderm disappear as the primary 

 mesoderm cells form themselves into flexor and extensor muscles 

 (ext.m, Fig. 136). The arched dorsal region of the egg begins to flatten 

 in consequence of the gradual digestion and diminution of the masses 

 of yolk stored up in the endoderm cells. The outer ends of these 

 cells, in which the nuclei are situated, gradually separate from the 

 yolky portions. These latter break up into rounded masses and are 

 gradually digested. The first place where the cytoplasm separates 

 from the yolk is in the dorsal surface of the mid-gut, just where the 

 proctodaeum impinges on it. Here a flat stretch of epithelium, the 

 "endodermal plate," becomes separatedfrom the yolk (end.p^ig. 135). 

 Soon the rostral spine begins to be differentiated between the anten- 

 nules in the head region. The gills appear under the branchiostegite 

 as outgrowths from the basal joints of the limbs. The two halves of 

 the caudal fork fuse to form a simple rounded telson. The ectoderm 

 everywhere sends inwards solid pegs ; they form the supports for 

 the tendons and ligaments of connective tissue which are formed by 

 the wandering cells. 



The embryo is now ready to break open the egg-shell and enter 

 upon its free life. For some time the store of yolk in the endoderm 

 suffices ; but gradually the extreme convexity of the dorsal hump dis- 

 appears, as the remaining store of yolk is used up, and the endoderm 

 cells shrink in size. The flaccid endodermic sac becomes indented 

 by folds, and is gradually fashioned into the complex structure of 

 tubes known as the adult liver. Its median portion persists as the 

 adult mid-gut. 



Just before the embryo hatches Eeichenbach was able to detect 

 the rudiments of the genital organs. These appear in the 14th, 15th, 

 and 16th segments in the dorsal region, and appear to consist of 

 rounded masses of cells, in each of which a lumen appears. The 

 masses seem to be arranged metamerically in accordance with the 

 segments, and at the hinder end of the rudiment of each side there 

 is to be seen a tube, which is presumably the rudiment of the genital 

 duct. Eeichenbach's imperfect observations, so far as they go, fit in 

 admirably with Sedgwick's results on Peripatus. 



OTHER CEUSTACEA 



We shall now take a brief survey of what is known of the 

 development of other Crustacea, and shall direct our attention to 



