lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



tomes) to give greater efficiency to body movement. The segmen- 

 tation of the ectoderm and the mesoderm then followed as a result of 

 the segmentation of the muscular system. The primitive coelomic 

 cavities were probably spaces formed' in the mesoderm for the accu- 

 mulation of waste products in the body fluid, to be discharged through 

 primitive nephridial tubules. The coelomic mesoblast, however, 

 formed also a secondary musculature that reinforced the primary 

 musculature, and which, in the higher arthropods, has completely 

 replaced the latter. Evidence that such has been the course of evolution 

 in the Articulata will be shown in the following discussion of the 

 early stages in annelid ontogeny ; but there still remains the question 

 as to the origin and nature of the primitive mesoderm, which antedates 

 metamerism. 



A study of the growth and differentiation of the annelid mesoderm 

 takes us into the later part of larval development, but to obtain light 

 on the origin of the middle germ layer we must go back to an earlier 

 ontogenetic stage. During cleavage of the annelid egg most of the 

 yolk remains consistently in the blastomeres situated on the vege- 

 tative surface of the blastula (fig. 5 A), with the result that, in the 

 64-cell stage, there are 8 large, yolk-filled blastomeres at the posterior 

 pole (B). These cells are designated by embryologists 4A, 4B, 4C, 

 4D, and 4a, 4b, 4c, 4d, since they comprise the so-called macromeres 

 of the fourth generation and the fourth quartet of micromeres. All 

 of them at this stage would appear to be endodermal, and at the time 

 of gastrulation they all become internal, owing to their overgrowth 

 by the ectoderm. Seven of them, in fact, give rise to purely endo- 

 dermal progeny, but the 4d cell will form in most cases both endo- 

 derm and mesoderm. The first cleavage of 4d produces two bilaterally 

 symmetrical cells, 4d^ and ^rf" (C), and these cells, in their immedi- 

 ately following divisions, give rise to a few very small cells (D, end), 

 usually regarded as endoderm cells, and a pair of large cells {MsT) 

 that are destined to produce the coelomic mesoblast, and hence con- 

 stitute the mesodermal teloblasts. (It is perhaps possible that the 

 small "endoderm" cells of this generation are the primary germ cells.) 



The common occurrence in the annelids of mesodermal teloblasts 

 derived from cells closely associated with the endoderm has given 

 rise to the idea that the coelomic mesoblast is of endodermal origin, 

 and for this reason it is often called the "endodermal mesoblast" to 

 distinguish it from the larval mesoblast, which is derived from the 

 ectoderm. In most animals the mesoderm is, one way or another, 

 associated in its origin with the endoderm, but among the annelids 

 there are many cases where its endodermal connection is not evident. 



