CIRCULATION, NUTRITION, AND RESPIRATION. 



13:3 



astonishing rapidity, in some cases at tlic rate of tlircc quarters of an incli in thirty-six 



Tiie distal extremity of the branch was its growing point, and here it terminated in a slight 

 enlargement which formed a cul de sac, in whose walls both cndoderm and ectoderm were dis- 

 tinctly difTerentiatcd (woodcut, fig. 5")). The somatic 

 fluid penetrated to the extremity of the branch. The 

 ectoderm formed a considerably thicker layer than the 

 endoilerm, and was composed of very distinct, nearly 

 spherical cells, with clear granular contents, and with 

 but little intercellular plasma. Numerous slightly 

 curved, cylindrical bodies, resembling undeveloped 

 thread-cells, appeared to lie free in the intercellular 

 ])Iasma, where they were scattered among the true 

 cells.' Under the action of acetic acid a distinct nu- 

 cleus was rendered evident in the ectodermal cells. 



The endoderm of the growing point was also dis- 

 tinctly composed of spherical cells in a very scanty 

 intercellular plasma. The endodermal cells, however, 

 were much smaller than those of the ectoderm, and, 

 instead of the clear contents of the latter, the endo- 

 dermal cells contained coloured granules. 



On following the cells of the ectoderm backwards 

 from the growing point, they were found to become 

 less and less distinct ; it was, indeed, plain that they 

 had undergone a metamorphosis, and at a short dis- 

 tance from the extremity no true cells could any longer 

 be detected in the ectoderm, which now presented in 

 a sectional view only an obliquely striated and granular appearance. The endoderm, on the other 

 hand, retained its distinctly cellular structure throughout. The perisarc, which on the proximal 

 part of the branch possessed considerable thickness, showed itself on the growing part as a scarcely 

 perceptible pellicle. 



I did not succeed in detecting anything like a process of cell-division in the ectoderm of the 

 growing point, nor could I satisfy myself that the cells became multiplied by endogenous forma- 

 tion of cell-broods within the cavity of the older ones. The appearances were, on the other hand, 

 rather in favour of the direct formation of the cells out of the plasma. 



The cells of the endoderm probably originate in the same way, but they appear soon to 

 become ruptured and to discharge their contents into the somatic fluid, which filled the cavity of 

 the growing stem. 



In certain other tissues, growth would seem to take place by cell-division. This, at all 

 events, appears to be the case in the epithelium of the umbrella of ObeUa (see below woodcut, 

 fig. 59, c). 



Growing extremity of an nilvcntitions branch of 

 Obelia dichotoma. 



a. Ectoderm overlaid by a delicate pellicle of 

 cliitine, und composed of distinct cells, where it occu- 

 pies the growing point ; b, endoderm ; c, ectoderm, 

 somewhat removed from the growing point, its cells 

 no longer evident ; rf, some of the cells from the ecto- 

 derm of the growing point, treated with acetic acid, 

 more highly magnified, and with thread-cell-like 

 bodies lying scattered among them. 



^ This structure recals that of certain young sponges, with the spiculae which are to form the 

 skeleton scattered among more or less individualised masses of protoplasm. 



