THE MESOBLASTIC BANDS. 69 



mesoblastic bands (mb.) had only a very small extension for- 

 wards (five sections in front of the blastopore). They consist of 

 bands of nuclei placed in the vacuolated protoplasm inter- 

 veuing between the ectoderm and endoderm. 



The mesoblastic bands gradually acquire a greater extension 

 forwards as the embryo and the blastopore increase in length. 

 They very soon become segmented — before they have reached 

 the level of the front end of the blastopore. This fact is 

 clearly shown by Plate II, figs. 23 and 24. The segments 

 so formed do not at first (Plate II, fig. 23) possess any dis- 

 tinct central cavity around which the nuclei are arranged. 

 By the stage figured in fig. 24, Plate II, in which five meso- 

 blastic somites are present, the cavities have appeared. 



The appearances presented by surface views are confirmed 

 by an inspection of sections. I have sections of embryos, in 

 which three solid somites could be made out on each side. 

 Plate VI, fig. 2, represents a transverse section through a 

 stage with one solid somite. The section is slightly oblique 

 and passes through the somite of one side only ; on the other 

 side the section passes in front of the mesoderm. Fig. 5 a — -f, 

 Plate VI, represents sections through an embryo of the stage 

 of fig. 25, Plate II, in which the somites had reached the 

 level of the anterior end of the blastopore. A distinct cavity, 

 around which the nuclei of the somites are arranged, is present 

 in the somites. By the close of Stage b (fig. 25, Plate II) 

 the somites of the anterior pair have reached their permanent 

 position in front of the blastopore. They are separated from 

 one another by the alimentary canal, which at this stage 

 extends in front of the mouth (Plate VI, figs. 4, 6 a). 



With regard to this development the following questions 

 which can only be settled, if they can be settled at all, by a 

 study of transverse sections, present themselves: — (1) How do 

 the mesoblastic bands grow forward before they have seg- 

 mented into the somites ? and (2) How do the somites, which 

 are established before the bands have reached the front end of 

 the blastopore, reach their final position? 



1. Do the bands grow forward independently of surrounding 



F 



