416 CASWELL GRAVE 



with, a factor that does greatly disturb some of the developmental 

 processes of the egg of Ophiura v/ill appear later. 



A segmentation cavity makes its appearance very early in 

 the segmenting eggs and, during stages in late segmentation 

 and early blastula, is somewhat eccentric in position on ac- 

 count of the more rapid multiplication and growth of cells situ- 

 ated at the animal pole than of those at the vegetative portion 

 of the egg (text fig. 1, F). 



BLASTULA AND MESENCHYME FORMATION 



The blastula of Ophiura, in the stage during which the mesen- 

 chyme cells are being transferred from a position in the epi- 

 thelial wall to one in the segmentation cavity, merits careful 

 description because of the clearness with which its constituent 

 cells show, by their form and structure and by their interrela- 

 tions, the effect of a force, or forces, which takes part in this 

 change in position of the mesenchyme cells. 



By reference to figure 1, which is a camera drawing of a me- 

 dian sagittal section of a blastula fixed in a sublimate-acetic 

 solution, it will be noted that the mesenchyme cells seem to 

 be pushed or squeezed from the wall of the blastula into the 

 segmentation cavity by a force exerted upon them from behind 

 and from the side, by the adjacent parts of the blastula. As 

 a result of this pressure the nucleus of each of the 'inwandering' 

 mesenchyme cells has taken the form of a cone having its apex 

 pointed away from the segmentation cavity. The small mass 

 of yolk-free cytoplasm by which each nucleus is surrounded 

 shows the effect of this pressure, even more than does the nu- 

 cleus, and in every case it has the form of an elongated strand 

 of material trailing behind its nucleus with its long axis perpen- 

 dicular to the lines of force of the lateral pressure. 



The membrane, by which the blastula is enclosed, is thrown 

 into wrinkles and irregularities in thickness over the area in- 

 volved in mesenchyme formation and thus also registers the 

 effect of coercive pressure and constriction. 



