MUSEUM OF COMPARATIVE ZOOLOGY, 15 



ous deeply stained band of cuticula. In Figure 11 it no longer appears 

 quite homogeneous, but is darker at some places than at others. The 

 ectoderm is here composed of cuboidal cells. At a later stage of devel- 

 opment the ectodermal cells have become very much flattened. A thin, 

 unstainable, more deeply lying cuticula has already begun to form, and 

 the outer deeply stainable cuticula is seen to be broken up into bits. 

 Figure 13 is from the adult body wall. The ectoderm is flattened. The 

 inner cuticula has attained a great thickness, and the outer cuticula is 

 represented by only a few deeply staining patches. One attains a simi- 

 lar result by studying the sui'face of a stained individual. Figure 10 

 shows the condition of the outer cuticula at intervals along the same 

 branch from the gemmiparous region a to a nearly adult region, d. The 

 bits of cuticula become more and more widely separated and smaller, as 

 I have already described in detail on page 7. Here, then, we have not 

 merely an interesting case of replacement of one cuticula by another to 

 meet the needs of the enlarged body wall by a method which has no par- 

 allel, so far as I know, in any other group of animals, but for the specific 

 purposes of our problem a criterion of growth of the body ivall quite as 

 satisfixctory as karyokinesis, and much easier of application. 



Let us apply this criterion in our attempt to answer the question, Is 

 that portion of the body wall lying between the neck of the polypide 

 and the points of origin of the pyramidal muscles (Plate VI. Fig. 63, 

 h-a, h-c) derived wholly from the neck, or is it merely the result of 

 interstitial growth of that part of the 'original cystid which was pre- 

 formed in the neck region 1 If the first condition is true, we should 

 expect to find no indications of the outer cuticula seci'eted by the tip of 

 the branch ; if the second, we should expect to find the outer cuticula 

 broken into bits, and underlaid by the inner lately formed cuticula. 

 Figure 63 shows clearly the deeply stained outer cuticula here sep- 

 arated into bits, and, to my mind, thereby proves that this part of the 

 cystid has had an origin similar to that of the rest of the body wall. 

 Moreover, a comparison of the portion of the section figiared with the 

 remainder (and this comparison has been made on many sections from 

 several individuals) shows that the parts of the cuticula about the neck 

 are indeed rather smaller and farther removed from each other than at 

 the opposite side of the bi-anch ; but the difi"erence in this respect is not 

 very marked, and may well only signify that there is a more rapid 

 growth of the body wall in the vicinity of the neck of the polypide than 

 at the opposite side. 



But how then do the points of origin of the pyramidal muscles come 



