MUSEUM OF COMPARATIVE ZOÖLOGY. 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 surface 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 wall quite as 
satisfactory 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, 
b-a, b-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? If the first condition is true, we should 
expect to find no indications of the outer cuticula secreted 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 
eystid has had an origin similar to that, of the rest of the body wall. 
Moreover, a comparison of the portion of the section figured 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 branch; but the difference 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 
