EARLY DEVELOPMENT OF HYDRA. 25 



whole cell. Rarely more than one is produced by one cell so 

 that the several processes constituting a group are formed from 

 contiguous cells. The ends of the process show a marked curve 

 away from the axis of the group. The growth outward of these 

 processes carries with them at their extremities the egg mem- 

 brane which remains a noticeable part of the embryo even at the 

 time it is set free from the mother. The ectoderm cells at this 

 time are very long and appear greatly distorted as a result of the 

 great change in form. The outer part of the ectoderm cells then 

 gives rise to a thin chitinous envelope as shown in Fig. 12, PI. II. 

 The cytoplasm within the envelope and near the extremities of 

 the projections becomes very much more granular than the 

 remaining portion, and takes a darker stain as a result. The 

 chitinous covering continues to thicken, and the protoplasmic con- 

 tents of the processes apparently give rise to the chitinous material 

 of which the spicules are composed, whether by a transforma- 

 tion of the protoplasm into the chitin or by a process of secretion 

 of the material of which it is composed, I am unable to state. 

 The appearance of the protoplasmic granules just within the 

 chitinous covering of the processes would indicate a deposition of 

 material at that point. As this chitinous envelope becomes 

 thicker, and of course harder, it gives rise to a very effective pro- 

 tective covering. From the above account it is evident that the 

 covering takes its origin from the outer part of the ectoderm cells ; 

 the basal portion containing the nuclei and a few pseudo-cells 

 remains apparently unchanged. Beneath the shell there is now 

 a layer of cells of somewhat smaller dimensions but otherwise 

 differing little from their condition before the formation of chitin. 

 From the outer part of these cells there is then secreted the thin 

 inner membrane, and the process is thus completed. 



This account agrees in general with that given by Brauer (91^) 

 for Hydra sp. ?. After the completion of the process of shell for- 

 mation the embryo remains attached to the mother by means of 

 two or three tough strands which are also somewhat chitinous. 

 They are apparently identical with those holding the egg after its 

 emergence from the ectoderm. In the meantime the shell be- 

 comes harder and sections show its characteristic laminate 

 structure, that is, the delicate lines which give it an appearance 



