PHORONIDEA. 



a small central cavity develops, and in this way a l)lastula forms 

 which at first is spherical and then becomes elongated in the direction 

 of the future longitudinal axis ; in this l^lastula, a vegetative (ento- 

 dermal) portion consisting of larger cells can be distinguished from 

 a small-celled animal portion. The longitudinal axis coincides with 

 the plane which divides the animal half from the vegetative half. 



A true invagination-gastrula then develops (Fig. 1), the blastopore 

 being originally oval (Fig. 1 C), l)ut its posterior portion becomes 

 slit-like and soon closes. The anterior part that has remained open 



persists as the oral 

 -'■' ■t) /■'-■'"""■-^ aperture of the 



larva (or, more cor- 

 rectly, as the oeso- 

 phageal aperture). 

 The cells near the 

 posterior closed part 

 of the blastopore are 

 found still later to 

 be actively dividing, 

 and, according to 

 Caldwell, they take 

 part in the formation 

 (if the mesoderm, this 

 l)eing especially the 

 case near a dei^ression 

 (Fig. 1 1), (i) at the 

 most posterior part 

 of the blastopore. 

 Caldwell compares 

 Phoronis in this re- 

 spect to the Yerte- 

 brata, and calls the 

 whole of this closed 

 portion of the blasto- 

 pore (Fig. 1 D) the pnnnfiv^ streaJc, and the depression appearing 

 in the latter the ^}r^^»^Y^i'e groove. 



Meanwhile, the embryo changes somewhat in shape. Its anterior 

 part becomes swollen (Fig. 1 D) as the first indication of the future 

 pre-oral lobe of the larva. The region of the primitive streak noA\- 

 lengthens greatly, so tliat the depression mentioned al)o\'e shifts 

 quite to the posterior end of the embryo. The surface upon which 



Fio. 1.— GastiMila staye of Plwronis. A, younger, B, older 

 stage ill optical section (after Metschnikoff) ; x, proto- 

 plasmic bodies in the blastocoele (niesenchynie cells ?). C 

 and D, ventral superficial aspects (after Caldwell). 

 C, stage with oval blastopore. D, stage with slit-like, 

 partly closed blastopore ; m, anterior open part of the same, 

 from which the mouth of the larva is derived ; g, posterior 

 pit-like dejiression of the ijriniitive groove ; a pre-oral lobe 

 is distinctly marked off at this stage. 



