544 PROCEEDINGS OF THE ACADEMY OF [I't'C.,, 



begins to invagiiiate in the region of the four small niacromeres. The 

 ectoderm pushes in and forms a small tube, which later becomes the 

 ])harYnx. Previous to this invagination the endoderm cells derived 

 from the divisions of 4c/^-^ and 4(/'- have formed a solid mass of cells 

 in the lower part of the embryo (fig. 36). Soon after the invagination 

 starts, the beginning of a lumen in the endoderm cells is apparent by the 

 separation of the cells (fig. 37). This lumen rapidly becomes large and 

 ciliated throughout (PI. XL, figs. 38, 39, text fig. 4). The canal is at 

 first bent towards the posterior side of the larva (fig. 38), but with its 

 further development and enlargement it pushes forward under the 

 ganglion (fig. 39). Distinct cell boundaries can seldom be made out 

 in the endodermal portions of the canal. The inner borders of these 

 cells surround the large yolk spherules. In many cases amoeboid cells 

 can be seen spread out on the surface of these yolk spheres. In the 

 oldest larvse which I was able to obtain (fig. 39) the canal showed no- 

 indications of the lateral branches which become evident in the adult 

 worm. In these larvae some of the reduced yolk spheres are still 

 jiresent. 



From the account I have given it will be seen that practically all 

 of the alimentary canal is derived from the two primary entoblasts, 

 4d^-^ and 4d^-". Certainly the larger portion of the canal has such an 

 origin. There are three other possible sources of a portion of the canal, 

 although no one of these forms any considerable amoimt of it. One 

 is that the pair of small cells, 4cF'^-^ and 4d^-\ derived from the further 

 division of the mesentoblast, may form a small portion of the endoderm. 

 The chief reason for suspecting this, is that these cells are formed and 

 remain exactly in the path of the future canal. The second possibility 

 is that the three anterior cells of the fourth quartet, 4a, 46, and 4d, may 

 contribute a small amount to the canal at a late period. I believe, 

 however, that such is not the case. The shrunken nuclei of these 

 three cells, which can be seen at the time the canal is forming (fig. 36), 

 indicate that these cells degenerate without dividing further. That 

 the shrunken condition of these nuclei is not a preparation for karyo- 

 kinetic division is, I believe, fully establislied by the fact that these 

 nuclei remain in this shrunken condition for a very long time. Their 

 al^ility to take up the stain gradually becomes less and less, and the last 

 that can be seen of them (fig. 36) shows a faint, very irregularly out- 

 lined nucleus, not at all resembling one about to divide by mitosis. 



The third possibility in this connection is that the four small macro- 

 meres may, instead of degenerating, contribute a portion to the 

 endoderm. The degenerating character of the nuclei of these cells and 



