EARLY DEVELOPMENT OF HYDRA. 9 



well as contributing considerably to the proportions of the egg 

 by the addition of a considerable mass of cytoplasm. 



It is evident from the above description that there is here an 

 unusual case of nuclear degeneration. Processes of nuclear 

 fragmentation have been described as occurring in the eggs of 

 other coelenterates, but in a correspondingly later stage in their 

 history and with ultimate results of an entirely different nature. 

 Hickson ('93) has noted nuclear fragmentation in the case of 

 Distichopora violacea ; Allen ('00) in Tubularia crocea ; Hargitt 

 ('04) in Eiidendrium and Pennaria tiarella. These cases are, 

 however, quite different in that they consist of a simple breaking 

 up of the nuclear elements into small parts without their entire 

 dissolution. Herrick has described nuclear fragmentations in 

 the Crustacea, viz., in Alpheus minor, A. saideyi and Homavus 

 americanus in which the nuclei constrict off small buds some- 

 what after the fashion of growing yeast. The process takes 

 place for only a limited time in the egg-nauplius stage, and in 

 definite regions. These cases differ widely from the one herein 

 described in which the nucleus breaks down entirely and eventu- 

 ally disappears in the cytoplasm of the ^%%. 



Soon after the &^^ has increased to a size six or eight times 

 that of its original proportions, the nourishing cells are taken up 

 in an entirely different fashion, and in a manner probably designed 

 for supplying the embryo with energy in the later stages of its 

 development. This process results in the formation of the so- 

 called pseudo-cells. In some cases it does not begin until the 

 &^^ is nearly one half its matured proportions. The beginning 

 of the process is probably dependent upon the presence within 

 the ^%% of some chemical compound which incites the changes to 

 be described. The production of this substance is probably 

 accomplished at different periods of development in different 

 eggs. 



Inasmuch as I have attempted to work out in some detail the 

 cytological changes involved in the formation of the pseudo-cells, 

 it may not be out of place to insert an outline of the opinions 

 held by some previous investigators with reference to their 

 origin. 



