MUSEUM OF COMPARATIVE ZOOLOGY. OO 



at which muscle fibres were arising. A similar series through a slightly 

 older bud gives for the same regious respectively 5, li, and 13 cells per 

 section. lu series through older buds, a rapid decline m the number of 

 these cells occurs so that at the stage of Figure 30 (Plate IV.) there is an 

 average of only about 3.1 cells per section through the bud, and about 

 2.2 immediately below. These reticulated cells are not very numerous 

 in the region of the bud at the time this is about to arise, as a look at 

 the sections Figures 3 and 4 shows. One finds reticulated cells in the 

 mesoderm at tlie tip, and most abundantly at a rather early stage in 

 the development of the bud. The number of these cells diminishes as 

 one leaves the young individual to pass into the next older of the same 

 branch. In the adult such cells are rather rare ; so rare, in fact, that 

 Kraepelin ('87), who studied with care the body wall of the adult indi- 

 vidual, makes no mention of them. Nevertheless they do occur in the 

 cells which are to go into the lateral branch (Plate II. Fig. 15), as well 

 as elsewhere on the body wall. The place in which one finds the reticu- 

 lated cells most abundant, however, is in the young lateral branches near 

 the time when the polypide bud is about to arise. Here every cell of the 

 mesoderm is greatly enlarged, and filled with the vacuoles (Plate VT. 

 Fig. 58). These are very apparent upon a surface view of the branches. 

 Reticulated cells occur not only in the mesodermic cells of the body 

 wall, but also in those of the polypide bud, which were, indeed, only 

 lately a part of the mural mesoderm (Plate III. Fig. 28, Plate VI. Fig. 

 56). Thus, in general terms, we may say that the reticulated cells of 

 the mesoderm are chiefly confined to regions in which there are young 

 buds developing; and since these arise at intervals only, there is a 

 periodicity in their appearance, — a time of maximum development 

 followed by one of decline, then one of reproduction of such cells in 

 the ends of branches culminating in another maximum, and so on. 



Turning our attention now more particularly to the structure of these 

 reticulated cells at the period of their best development, we find (Plate 

 VI. Figs. 56, 57, 59) that they possess a large nucleus lying at the 

 deep end of the cell and containing a relatively large nucleolus, and that 

 this is surrounded by a granular protoplasm with included vacuoles. It 

 is very common to find the nuclei in various stages of division, and thus 

 it is frequently seen as a mass of chromatic substance without any nu- 

 clear membrane or nucleochylema. The vacuoles, which in the more reg- 

 ular cells lie in a semicircle nearly peripheral (the nucleus being at the 

 centre), are highly variable in number, some of the cells containing as 

 many as 20 to 30. They often appear as perfectly clear homogeneous 



