330 HVLSO.V. [Vol. IX. 



nucleate masses, as in the previous stage. Most of the mes- 

 entoderm cells have, like the ectoderm cells, plump bodies full 

 of fine yolk granules, but there are some with smaller slender 

 bodies, in which there is but little yolk and which begin to 

 assume the appearance of the spindle-shaped cells, so abundant 

 in the older embryo (compare PI. XXI, Fig. 8i). The pieces 

 of spicules found here and there in embryos of this stage were 

 already present in the gemmule before it began to break up. 

 It will be noticed in this stage, Fig. 75, that the ectoderm 

 cells are, in many instances at any rate, connected with the 

 mes-entoderm cells by line terminal processes. This connection 

 probably continues to exist in the later stages, but I did not 

 satisfactorily demonstrate it. 



The ectoderm cells which already form a distinct layer in 

 Fig. 75 divide in planes vertical to the surface, and become 

 long slender columnar cells. These slender columnar cells 

 form for a time a uniform investment for the whole embryo, 

 PI. XX, Fig. 76, though later they flatten out over one pole. 

 In PI. XXI, P'ig. 76, is shown a small part of Fig. 76, more 

 highly magnified. The rounded multilobed masses of the 

 earlier stage are no longer found in the mes-entoderm, which 

 now consists only of separate cells. 



While the embryo is still in the body of the mother and 

 surrounded by its capsule, the ectoderm cells over one of the 

 poles flatten out, while elsewhere they develop cilia and become 

 deeply pigmented. In PI. XX, l-'ig. ^7, is shown a section 

 through this pole of the embryo at a stage just before the 

 flattening has begun. The ectoderm cells over the general 

 surface have flagella, and nuclei near their lower ends, the 

 nuclei forming a zone several layers thick. The ectoderm 

 cells at the pole, however (ec. im-p. J>.), are not quite as slender 

 and have no flagella. Their connection with the cells of the 

 mes-entoderm is still obvious. These cells gradually flatten 

 until, by the time the embryo leaves the body of the mother, 

 they have assumed the character shown in PI. XXI, Fig. 78 

 {ec. iiii-p. J>.). The mes-entoderm in the stage shown in Fig. jy 

 is much as in the earlier stage, with the exception that a 

 number of spicules are now scattered through it, all of them 



