92 BNTOPROCTA. 



type. It is, however, not altogether regular, since the two-celled stage, 

 at which the two blastomeres are not quite equal in size, is followed 

 first by a three-celled stage, and only later by one showing four cells, 

 this latter stage being reached by the separation from each of the 

 two primary blastomeres of a somewhat smaller cleavage-sphere, near 

 the animal pole. As the blastomeres of the animal pole now rapidly 

 increase in number, and a central cavity appears, a blastula stage 

 arises (Fig. 43 A) ; in this, the cells of the vegetative half are 

 remarkable for their size and granular character. 



Further development leads to a formation of a true invagination 

 gastrula (Fig. 43 B). The vegetative half of the embryo first 

 flattens and then becomes invaginated towards the animal pole, the 

 cleavage-cavity consequently being almost obliterated. The blastopore 



Fio. 43.— Three stages in the embryonic development of PediceMna echinata (after Hatschek, 

 from Balfour's Text-book). A, blastula stage preparatory to gastrulation. Optical section 

 seen from the side. B, gastrula-stage. Optical section from above. C, later stage, after the 

 closing of the blastopore, seen from the side, a.e, archenteric cavity ; ep, ectoderm ; hy, 

 entoderm ; me, mesoderm cells ; s.c, cleavage-cavity. 



closes in the form of a longitudinal slit corresponding to the median 

 plane, and to the middle of the future ventral surface. A flattening 

 of the ventral side can already be noticed (Fig. 43 C). At. that end 

 of the slit-like blastopore which can be directly observed to give 

 rise to the posterior or anal end of the adult, two symmetrically 

 placed cells (me) now appear; these have retained the primitive 

 character of cleavage-spheres, and are not yet externally covered by 

 the ectoderm These are the primitive cells of the mesoderm. They 

 are soon completely grown over by the ectoderm (Fig. 43 C), and 

 then lie in a space between the ectoderm and the entoderm, which is 

 derived from the cleavage-cavity, and is considered to be the primary 

 body-cavity. This cavity develops further during the following 

 stages, and gradually becomes filled with mesodermal elements. 



