CKANGON VULGARIS. 25 



pigment patches, as well as on the ocellus, are very frag- 

 mentary, and concerning them and the further development 

 of the nervous system, I have at present nothing further 

 to offer. 



A comparison of the development of the nervous system 

 of Crangon with that of Astacus, as described by Reichen- 

 bach, is not an easy task, since my sections show but few 

 of the structures he figures and describes. In that form 

 (cf. Reichenbach, J 86, pp. 64-65) , the primitively postoral 

 ganglia of the brain move forward at a much earlier date 

 than they do in Crangou, and the fibrous portions of the 

 chain are correspondingly precocious in their appearance. 

 Nothing like the middle cord and thrice divided lateral cords 

 of Reichenbach appear in Crangon, at least until after hatch- 

 ing, nor have I seen any division of the first anteunal gan- 

 glion to form the " Seitenanschwellung." 



MESODERM. 



In the previous part of this paper, the mesoderm was 

 described as arising from the anterior and lateral margins 

 of the blastopore. It retains its primitive condition and 

 exhibits no differentiation until about the stage A. Then 

 as shown in sections 29, 30 and 31, it has increased consid- 

 erably in extent and has crowded itself between the ecto- 

 and entoderm, reaching to a point a little behind the blasto- 

 pore or anus. In front, it has spread out beyond the region 

 of the abdominal flexure. A series of transverse sections at 

 this stage show that the layer is essentially circular in out- 

 line, except for two lobes which reach forward a short dis- 

 tance on each neural band. Fig. 31 represents a section 

 passing through the neural band of either side, the region 

 of which may be recognized by the columnar character of 

 the epithelial cells, while beneath each band may be seen 



ESSEX INST. BUIXBTIN, VOL. XXI 2* 



