704 PROCEEDINGS OF THE ACADEMY OF [Oct., 



stomach projects a short diverticuhim, the rudiment of the intestine 

 (int. 1.). 



The proctodseum appears, at the time that the stomodseum has 

 assumed its final position, as a simple thickening of the ectoderm at the 

 posterior end of the body (fig. 59, pr.). At a later stage a small depres- 

 sion appears in this thickening (fig. 60, pr.). The proctodseum prob- 

 ably forms only the terminal portion of the intestine, or anus. Fusion 

 of the proctodseum and stomodseum with the entodermal portion of 

 the alimentary tract does not take place until very shortly before 

 hatching. 



The rudiment of the brain appears early. Just after the blastopore 

 has closed, the ectoderm at the anterior end of the embryo is thicker 

 than elsewhere, presenting the appearance shown in optical section 

 in fig. 58. The cells of this region multiply rapidly, and the ectoderm 

 here soon becomes many-layered (fig. 59). At a later period (figs. 60 

 and 61) a transverse band of nerve fibres (br.com.) is seen below 

 and in contact with the ectoderm at this point, while the nuclei of the 

 deeper layers have become very numerous. The latter undoubtedly 

 constitute the nuclei of the ganglion cells, while the band of nerve 

 fibres constitutes the commissure connecting the lateral lobes of the 

 brain of the adult. The brain is now essentially similar to that of the 

 adult, which remains throughout life in contact with the ectoderm of 

 the head. 



The further changes before hatching consist principall}^ in the elonga- 

 tion of the body and the formation of somites. The rapid growth of the 

 ventral plate causes the embryo to bend strongly to^^•ard the dorsal 

 side, as is shown in fig. 60, though here the flexure is not nearl}^ so great 

 as it becomes at a still later period. This rapid growth of the ventral 

 ectoderm is apparently not compensated for by a corresponding 

 growth of the dorsal ectoderm until shortly before hatching. The 

 head is separated from the trunk by a constriction at a stage corre- 

 sponding to that illustrated in fig. 59, that is, as soon as the stomodseum 

 has taken up its final position at the anterior end of the embryo. The 

 trunk segments do not appear until the body begins to elongate, when 

 the constrictions which mark them off appear successively from in 

 front backward. In the embryo shown in horizontal section in fig. 

 61, one trunk segment, the first, is clearly shown, while two more are 

 indicated posterior to this one. 



