PHORONIDEA. 



D 



a small central cavity develops, and in this way a blastula forms 

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

 of the future longitudinal axis; in this blastula, 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), but its posterior portion becomes 

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



persists as the oral 

 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 

 of the mesoderm, this 

 being especially the 

 case near a depression 

 (Fig. 1 D, g) at the 

 most posterior part 

 of the blastopore. 

 Caldwell compares 

 Phoronis in this re- 

 spect to the Verte- 

 brata, and calls the 

 whole of this closed 

 portion of the blasto- 

 pore (Fig. 1 D) the primitive streaJi, and the depression appearing 

 in the latter the primitive 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 now 

 Lengthens greatly, so that the depression mentioned above shifts 

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



OL^ 



sr 



Fig. 1. — Gastrula stage of Phoronis. A, younger, B, older 

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

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

 and D, ventral superficial aspects Rafter 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 depression of the primitive groove ; a pre-oral lobe 

 is distinctly marked off at this stage. 



