36 BULLETIN OF THE 



spaces, but more frequently at this stage contain a spherical body, which 

 frequently tills the entire vacuole and is more refractive than the sur- 

 rounding plasma (Vig. 59). Not unfrequently one sees a less refractive, 

 clear space, surrounding the highly refractive body (Fig. 57). 



The description just given corresponds to the condition seen in a ter- 

 minal branch whose polypide has attained the development of that shown 

 in Figure 28 (Plate III.). At the time immediately preceding the ori- 

 gin of the bud, the cuboidal cells of the mesoderm show traces of vac- 

 uolation, but their form and size have suffered no appreciable disturbance. 

 This vacuolation of cells proceeds hand in hand with the development of 

 the bud, and one first notices the homogeneous, highly refractive bodies 

 in the vacuoles when the bud is well established. At about the time the 

 alimentary tract has become formed, the reticulated cells begin to show 

 signs of degeneration. The highly refractive bodies have disappeared, 

 and the skeleton of the cell which remains becomes very irregular. As 

 already stated, the number of reticulated cells also decreases, until, at 

 about the time of " rotation " of the polypide, there are few reticulated 

 cells in the mesoderm, but these few are filled with vacuoles and their 

 highly refractive bodies. 



The conditions of the mesodermal cells at the tip are slightly different 

 from those found elsewhere. Usually, instead of many small vacuoles, 

 one finds only one or two which fill almost the entire cell, — sometimes 

 perfectly homogeneous in structure, sometimes containing small highly 

 refractive granules. 



These appearances I believe to be explicable only upon the assumption 

 that the mesodermal cells are capable, at the time at ivhich the young poly- 

 pide is arising, of imbibing the fluids of the body cavity and storing them 

 %ip for the piirpose of supplying the rapidly groxoing cells of the bud with 

 nutrition. It is desirable to show reasons for believing, first, that the 

 contents of these cells ai'e nutritive matter ; secondly, that this has been 

 taken up from the body cavity; and, thirdly, that it is supplied to tlie 

 bud for its nutrition. 



it must be admitted that the strongest argument for the belief that 

 these are absorbing cells is derived from a comparison of the appearances 

 which we find in these cells with those described for Protozoa, and by 

 Metschnikoff ('83, Taf. I. Figs. 18-35) for mesodermal trophic cells. 

 At the same time, it must be acknowledged that similar cells are found 

 in other cases where the function is believed to be not ingestive, but 

 excretory, as in the chlorogogen cells of Annelids, as shown by Kiiken- 

 thal ('85), Eisig ('87, pp. 751-762), and others, and indeed even in the 



