226 RANDOLPH. [Vol. VII. 



IV. Differentiation of Regenerated Tissue. 

 I. Ectoderm. 



At an early stage the ectoderm begins to increase in thick- 

 ness in the ventral and lateral regions. The nuclei arrange 

 themselves in a very definite way into a number of groups, — 

 five on each side of the median line. Of these the two ventral 

 cell masses are the first to appear, and they are the foundations of 

 the future ventral nerve cord. The layers of cells arrange them- 

 selves into two groups each approximately hemispherical in sec- 

 tion and which are in contact at the inner surface of the ecto- 

 derm (Fig. 8). The two foundations gradually unite and nerve 

 fibres appear near the inner surface in small bundles, which 

 subsequently grow larger {Fig. lo). The developing nerve cord 

 gradually leaves the ectoderm from which it arose, but retains 

 in its form traces of its paired origin. A thin layer of meso- 

 dermic tissue (circular muscles) eventually insinuates itself be- 

 tween the ventral nerve cord and the ectoderm which lies 

 beneath (Figs. 9, 10, 11, 12, 13). 



The fifth ectodermic foundation on each side gives rise to 

 the dorsal setae. From the fourth foundation arises the lateral 

 nerve line whose cells lie free in the body cavity, but by means 

 of fibres retain their connection with the ectoderm. 



The second and third foundations on each side (Fig. 8) are 

 evidently connected with the development of the nephridia and 

 the ventral setigerous glands, but to just what extent I am 

 unable to discover (Fig. 14). In the introductory part I have 

 already alluded to the small size of the cells and the contracted 

 space as productive of greater obscurity than is to be found in 

 similar stages of developing embryos of related forms. It seems 

 to me, however, not improbable that a form may be found that 

 will show clearly in its development the extent to which the 

 second cell row takes part in the formation of the nephridia. 

 If the part^ that Meyer assigns to the "primary mesoderm" 

 proves to be general for all Annelids, the nephridia will then be 

 found to be composed of three kinds of tissue. In this work 

 I am not aware of anything for or against this special point. 



^ See under Differentiation of Mesoderm. 



