1 88 WILSON. [Vol. I. 



band. As the blastopore narrows, the two cell-rows extend 

 forwards and upwards, and finally meet each other in the median 

 line above the mouth. The middle part of each row is at the 

 same time carried downwards, so as to lie at the side of the 

 body, somewhat towards the ventral aspect. Each row has, 

 therefore, a double curvature (the embryo now being nearly 

 spherical), passing downwards and forwards from the " meso- 

 blast," and then forwards and upwards to meet its fellow above 

 the mouth. As growth progresses the rows (now widened into 

 the mesoblastic bands) grow towards each other, and finally 

 unite just behind the mouth. From the initial point of union 

 concrescence proceeds regularly backwards throughout the 

 whole period of embryonic life, and is completed about the 

 time of hatching. During this process the bands constantly 

 increase in width until they join each other along the whole 

 dorsal median line, and thus completely surround the aliment- 

 ary canal. 



The six anterior teloblasts, viz., the neuroblasts, nephroblasts 

 and the "lateral teloblasts" {X), can first be distinguished 

 with certainty in spherical embryos towards the end of invagi- 

 nation. At this period they have the same arrangement as in 

 later stages, but lie in the ectoblast, extending to the surface of 

 the body. Each gives rise to a row of cells that can be traced for- 

 wards a short distance, and then is lost amongst the surrounding 

 cells. In later stages these teloblasts are gradually crowded be- 

 low the surface by adjoining ectoblast cells, though they always 

 remain embedded in the ectoblast, and sometimes reach the 

 surface in stages as late as that shown in Fig. 3. There is no 

 evidence that they are originally formed below the ectoblast, and 

 are afterwards pushed out to the surface. The only interpreta- 

 tion that I can put upon these observations is, that 7iot only 

 the neuroblasts, but also the nethroblasts and ^^ lateral teloblasts^' 

 are modified ectoblastic cells, 



VII. Summary. Comparisons. 



It is not my intention to give at present a full discussion of 

 the observations outlined in the preceding pages, but it seems 

 desirable to call attention to their bearing on certain mor- 

 phological discussions relating to the origin and homologies of 



