SOME PROBLEMS OF ANNELID MORPHOLOGY. 65 



It is not my intention to review the various theories 

 that have been put forward in place of the colonial 

 theory. Some of them are exceedingly ingenious ; none 

 of them are adequate explanations of metamerism. But 

 I wish to show that the study of this question is very 

 closely bound up with that of certain others which need 

 to be carefully studied in the embryology of annelids, 

 and which offer a very inviting field for investigation. 

 The segmentation of the trunk first arises in the inter- 

 nal parts of the embryo — i.e., in the mesoblast — and 

 upon this internal segmentation the external segmenta- 

 tion is, as it were, moulded. Now, the mesoblast, as 

 every embryologist knows, arises in all annelids in two 

 separate lateral masses (mesoblastic bands) that extend 

 lengthwise along nearly opposite sides of the trunk and 

 sooner or later join each other both in front and behind, 

 so as to form, as it were, a longitudinal ring, lying be- 

 tween the two primary germ-layers. In all cases the 

 bands grow mainly at their posterior ends, where, in 

 many cases, each terminates in a large pole-cell (or *'telo- 

 blast ") from which the entire band is derived (see Fig. 6). 

 As development proceeds the two bands widen out, 

 forcing their way between the ectoblast and entoblasf, 

 and ultimately grow together along the median line, 

 first below and afterwards above the alimentary canal, 

 which is thus entirely surrounded (Fig. 3, A., B., and C). 

 Metamerism, I repeat, first appears in these mesoblastic 

 bands, and is only secondarily extended to the other 

 parts. And it is interesting to observe, further, that 

 the segmentation of the bands is perfectly distinct long 

 before they unite in the median line. Each somite, 

 therefore, is formed by the union in the middle line of 



