12 BULLETIN OF THE 



those of the adjacent body wall. Have they been so ever since they were 

 derived from the tip, or have they secondarily become so 1 I believe 

 that these cells have never been flattened pavement epithelial cells, for 

 the following reason. All ectodermal cells of the body wall near the tip 

 are cuboidal ; if these cells have only secondarily acquired this form, 

 they must have passed through a stage in which they were flattened epi- 

 thelium. Now, if these cells could be distinguished by greater thickness 

 from the cells of the surrounding body wall, at a time at which the lat- 

 ter cells had only just begun to emerge from the cuboidal condition to 

 become difi"erentiated into the pavement epithelium of the body wall, it 

 would follow that, even though they had secondarily increased in size as 

 a result of an impulse preparatory to evagination, and even though they 

 would have been at a stage only a very little earlier indistinguishable 

 from the other cells of the body wall, yet they would never have passed 

 in this case through a flattened condition, because at a stage only a very^ 

 little earlier the whole body wall was composed of cuboid cells. 



The conditions which I have set as the criterion of our problem are 

 fairly realized in Figure 17, which represents a portion of the body wall 

 of a median branch which extends from the gemmiparous I'egion above to 

 the thickened body wall of the nascent lateral bud below (gm. L). It will 

 be seen, by a comparison of the body wall of this region with that shown 

 in Figure 19, which is taken from the same individual farther from the 

 tip, that even the most differentiated part of the body wall of Figure 17 

 is in a relatively indifferent condition as compared with the pavement 

 epithelium of the ectoderm of Figure 19, in which the mesoderm, indeed, 

 has become so thin and insignificant as scarcely to be visible. We may, 

 therefore, maintain that the ectodermic cells of the body wall have only 

 just begun to lose their cuboidal condition to become pavement epi- 

 thelium, and therefore conclude, in accordance with the argument just 

 presented, that the cells of the lateral bud {gm. l.) have never passed 

 through a stage in which they were flattened epithelium. It is evi- 

 dent, also, that the Anlage of the second lateral bud is also derived 

 from near the tip, because, as in Figure 20, we find two lateral regions of 

 cuboidal cells. 



5. Development of the Body Wall. 



It is, of course, almost impossible to gain direct evidence upon the 

 place of origin and method of development of the body wall, and one is 

 therefore forced to the collection and weighing of circumstantial evi- 

 dence. Braem ('90, pp. 127, 128, 131) believes that the body wall (the 



