92 



BNTOPROCTA. 



type. It is, however, not altogether reguhxr, 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 tlevelopnient of PediceUina cchinata (after Hatschek, 

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

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

 closing of the blastopore, seen from the side, a.e, arehenteric 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 pi'wiifive cells of t/ie 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. 



