584 CUESSWELL SHEARER. 



From Text-figs. 27, 28, which represent sections through 

 the growing bands and liead-kidneys of the four-day trocho- 

 phore, it will be seen that there are numerous mesenchyme 

 cells about the head-kidneys which could hardly have arisen 

 from the coelomesoblast cells (cce.m.), which, moreover, 

 show no evidence of having recently divided. I have 

 examined a large number of such sections without observing 

 in a single instance the division of these cells to form mesen- 

 chyme. 



Treadwell (43) holds the view that this separation of the 

 mesoderm in Annelids into apparently distinct portions is 

 only a mechanical result of development, but the varied con- 

 ditions nnderwhich a larval mesenchyme is present in Annelids 

 seems to me to be against this view. Treadwell (43) has 

 pointed out that we are forced to believe in two non-homolo- 

 gous sets of larval mesenchyme, the one arising from the 

 ectoderm as in Thalassema and Podarke, and. the other 

 from the anterior ends of the germ-bands, as in Nereis and 

 Ln m b r i c u s . ^JMiese two sets do not, as a rule, exist together. 

 " On the other hand," he says, " no one has proved, as far as 

 I know, that no ' mesenchyme ' arises from the germ-bands in 

 cases Avhere a larval mesenchyme exists." I have attempted 

 to show that in Eupomatus, where a larval mesenchyme 

 exists, no evidences of its origin from the bands can be 

 observed, and the main result of my work has been to empha- 

 sise the distinction between ecto- and coelomesoblast. I have 

 already considered in the "Introdnction" whether we are 

 justified in laying any stress on this point. In Annelids we 

 are at least certain that this separation seems general and 

 definite. 



Summary. 



Segmentation results in a round blastula with a very 

 reduced blastocele. Invagination produces at first an almost 

 spherical gastrula. But this soon begins to assume the conical 

 shape of the early trochophore. The blastopore, which is 



