THE CRAYFISH 145 



Turning now to the internal changes, fig. 34, B, shows 

 the relatively small size of the embryonic area at the time 

 when the mesenteron is still a closed sac of relatively small 

 extent. The thoraco-abdominal swelling is shown, traversed 

 by the proctodaeum, prc\ the latter is in contact with the wall 

 of the mesenteron at its lower end, but has not yet established 

 communication with it. The commencement of the stomodaeal 

 invagination is seen at st, and the mesoblast appears as a 

 number of scattered cells lying below the epiblast of the 

 embryonic area. The mesenteron has only absorbed a small 

 part of the yolk, and the hypoblast cells are comparatively 

 small. In fig. 34, F, great changes have taken place. The 

 embryonic area is much larger, and the cephalic and thoraco- 

 abdominal regions are well marked. The stomodaeum and 

 proctodaeum have increased to a marked extent, and the 

 latter touches but does not yet open into the mesenteron. 

 The rudiment of the cerebral ganglion is conspicuous, and 

 the mouth is overhung by a large labrum. The anterior 

 thoracic region is distinctly segmented, as also is the thoraco- 

 abdominal outgrowth, the latter being now bent forwards 

 nearly as far as the mouth. The segmentation, however, 

 affects the epiblast only, and is expressed in the section by 

 the epiblastic thickenings which will give rise to the ventral 

 chain of ganglia. The limbs do not appear, as the section 

 passes through the middle line of the body. The mesoblast 

 has no share in the segmentation, consisting as it does of 

 numerous scattered cells lying between hypoblast and epiblast ; 

 but posterior to the proctodseum a certain number of mesoblast 

 cells have united to form the walls of a hollow vesicle, the 

 heart. The hypoblast cells have completely absorbed the 

 yolk, and have increased enormously in size ; their external 

 nucleated ends are in contact with the epiblast over the upper 

 two-thirds of the embryo, and their inner ends, swollen by the 

 yolk they have absorbed, delimit the now spacious mesenteric 

 cavity. The latter, the mid-gut, has at this stage attained its 

 largest size ; as development progresses, the yolk stored up 

 in the hypoblast cells is withdrawn for the nourishment of 

 the other tissues of the growing body, and the mid-gut 

 becomes relatively smaller and smaller. 



In the stage shown in fig. 34, F, there is a band of meso- 

 blast cells lying dorsal to the proctodaeum in the thoraco- 



II. K 



