362 L. DONCASTER. 



9 — 12) it is seen that no ccfilomic space is present, and that 

 the mesoderm forms two strands running through the embryo 

 and separated from one another by the endodermic septum. 

 These mesodermic strands appear in cross-section roughlj^ 

 circular in outline, and near the circumference the cell 

 boundaries are well marked ; but it is seen that the nuclei are 

 no longer at the inner ends of the cells, and that beyond them 

 the space which in early embryos was free and unoccupied is 

 now filled with cell-substance, in which the boundaries 

 between the separate cells are very indistinct. This condi- 

 tion appears to have been brought about, firstly, by the com- 

 pression of the whole embryo and consequent reduction of its 

 cavities; and secondly, by the sinking of the mesodermic 

 nuclei towards the bases of the cells, that is towards the 

 plane separating the mesoderm from the ectoderm. The cell 

 protoplasm of the centre of the mesodermal strands, that is 

 within the ring of nuclei, is of a much looser and more watery 

 character than that near the circumference, which probably 

 accounts for the lack of distinction between the cells ; in 

 embryos in which the cytoplasm is less well preserved it tends 

 to form a mass of strands between which are clear spaces. 

 That this is not due entirely to faulty fixation and consequent 

 maceration is certain from the study of the living embrj^o, in 

 which an exactly similar appearance is seen at this stage as 

 was described in Section III. The basal part of the somatic 

 mesodermal cells is composed of much firmer protoplasm, 

 and the lines separating the cells can be seen distinctly at 

 this stage, and also when the embryo is more advanced. As 

 development progresses the nuclei become aggregated in a 

 dorsal and ventral mass on each side, while in the lateral 

 areas they disappear or become very scarce. Between the 

 nuclei and the base of the cells the protoplasm is becoming 

 modified, so that it now takes a deep brown stain with osmic 

 acid (figs. 13, 14), and a longitudinal section shows that the 

 cells are becoming elongated in the direction of the animal's 

 length. The study of the larva after hatching shows that 

 these dorsal and ventral groups of cells of the somatic meso- 



