MUSEUM OF COMPARATIVE ZOOLOGY. 117 



stage of development, this character is still retained by much of its 

 tissue. (See, for example, Fig. 11, fun., Plate IX.) 



The great retractor and rotator muscles have, I believe, like the funic- 

 ulus, a double origin. They arise from the outer layer of the bud, on 

 the one hand, and from the ccelomic epithelium on the other. The 

 first indication of the differentiation of the muscle cells consists in a 

 disturbance in the upper lateral edge of the outer layer of the bud at 

 about the stage of Figure 17, Plate III. This is shown in dorso-ventral 

 sections through this region (cl. mu., Figs. 24, 26, Plate IV.). Later, tlie 

 disturbance becomes more marked, and cells having a semi-amoeboid 

 character appear to be proliferated (cl. mu., Fig. 33, Plate IV.), and to 

 migrate from the bud towards the ccelomic epithelium. During this 

 process the cells of the latter layer also are active, and some of them, 

 elongating, reach towards the young polypide, as seems to be clearly 

 shown at cl. mus., Figure 54, PUite VI. It is significant that, since each 

 of the two upper lateral edges of the bud lies near a radial partition, the 

 muscles also are always formed in close proximity to one (disep, r., Fig. 54, 

 Plate VI. ; Fig. 30, Plate IV.). It will thus be observed that, both in the 

 case of the funiculus and of the muscles, the end which is attached to 

 the wall of the colony arises at a point which is remote from that of its 

 attachment to the adult. The migratiim to the later positions will be 

 treated of farther on. (See page 141.) 



6. Origin of the Body-wall. — As already shown (page 104), the 

 body- wall of the individual of a Cristatella colony includes not only 

 the endocyst of authors, — the roof and the sole, — but also the radial 

 partitions. 



Braem ('88, pp. 506, 507) concludes " dass die polypoide Knospen- 

 anlage . . . nicht allein das Polypid nebst den Tochterknospen liefert, 

 sondern dass auch die zugeliorigen Cystide ans ihr und zwar aus ihrem 

 Halstheil entwickelt werden." I believe that a portion of the " cystid," 

 or body-wall, is thus formed in Cristatella, but not the whole. 



If one compares the relations of the polypide to its daughter bud in 

 Figures 3 (Plate I.) and 17 (Plate III.), and reflects that later the daugh- 

 ter bud is to be found still farther from the mother bud, he is forced to 

 one or the other of two conclusions : either the young bud is pushed 

 from the mother by a proliferation of cells from the neck of the polyp- 

 ide without causing an increase in the length of the body-wall itself, 

 or there is an actual increase in the length of the body- wall, produced 

 either by the proliferation of cells already existing in it, or by the addi- 

 tion and subsequent proliferation of cells from the neck of the mother 



