THE DEVELOPMENT OF THE EARTH-WORM, 223 



would regain almost immediately its original volume, by the 

 aid of an extraordinarily energetic nutritive change. Now, 

 the cells produced in such a way from the large ones cer- 

 tainly would not be placed elsewhere than in the mesoderm, 

 and would form a part of it. I say, a part, because another, 

 and I believe the larger part, certainly has a different origin. 

 It has been explained above how the ectoderm cells trans- 

 form themselves into wide and flat plates ; this is true for 

 the dorsal and ventral surfaces, but the cells of those tracts 

 of ectoderm which cover the cords of mesoderm either keep 

 their longer or shorter prismatic or cylindrical shape or 

 recover that form after having been depressed before the 

 mesoderm comes to raise them (figs. 11a, 113, 12, 13, ecc). 



Now, while the larger number of the ectoderm cells show 

 little activity, and appear not to divide, except when their A 

 is no other way to prevent the interruption of continuity of 

 the external covering of the embryo, those which cover the 

 middle layer are in a state of the most rapid reproduction. 

 The newly made cells do not become employed in the 

 enlargement of the surface, but losing little by little their 

 connection with the layer from whence they took origin, 

 they force themselves inwards, when^ they unite with the '>^e/> 

 cells of the mesoderm. This relation appears to me to be 

 very easily and clearly recognisable. Sections, especially 

 transverse ones, show how, here and there, the line of 

 demarcation between the ectoderm and mesoderm disappears 

 altogether, while in other parts of the same embryo it is 

 very evident. It is impossible to decide whether certain 

 cells belong to the external or to the middle layer ; indeed, 

 it sometimes seems that the covering of the two cords is 

 folded inwards round their proximal margins, cells of the 

 external layer in this manner placing themselves below the 

 already formed elements of the mesoderm. But the direct 

 production of mesoderm cells from the external layer lasts 

 only a short time. With the progress of development a very 

 distinct demarcation becomes established between the two 

 layers, and the very important increase which the mesoderm 

 henceforward undergoes is produced solely by the multiplica- 

 tion of its own proper cells. 



On the other hand, with the greatest attention, I have 

 not been able to discover the least sign of the endoderm cells 

 participating in the formation of the middle layer, and as in 

 the stages under consideration their relative positions are 

 very clear and distinct, I do not hesitate to say that the 

 internal layer has no share in the formation of the mesoderm. 

 But how can this be ? I have admitted, at least for a part of 



