EMBRYONIC DEVELOPMENT. 



the blastopore is situated may be defined as the ventral surface 

 and the opposite as the dorsal surface. 



The mesoderm, according to Caldwell (No. 2) forms in a very- 

 peculiar way. In the region corresponding to the most anterior 

 part of the primitive streak, the entoderm-sac shows two lateral 

 pocket-shaped outgrowths (Fig. 2 A, d), at the base of which meso- 

 dermal elements {in') become detached through the proliferation of 

 the entoderm -cells. When a considerable number of these meso- 

 dermal elements have been formed, they arrange themselves into a 



Fig. 2.— Formation of the mesoderm in Phoronis (after Caldwell). A, transverse section 

 through the anterior part of the embryo, showing the origin of the mesoderm. B, trans- 

 verse section through the oral aperture of an older stage. C, horizontal section through 

 an embryo, showing the formation of the posterior coelomic sacs (m")\ only parts of 

 the anterior sacs (to') are cut through, ec, ectoderm ; en, entoderm ; d, paired archenteric 

 diverticula; g, Binall pit at the posterior end of the primitive streak; m', anterior, m", 

 posterior pair of coelomic sacs. 



pair of sacs, each enclosing a coelomic cavity (Fig. 2 B, in). This 

 formation of paired coelomic sacs by the proliferation of cells from 

 the lateral walls of the archenteron may, perhaps, be traced back 

 to the type in which the enterocoeles arise through folding of the 

 archenteron. Further back, single mesoderm-cells become separated 

 from the primitive streak and pass into the space between the 



