The Development of Phascolosoma. 103 



Growth of the Somatic Plate. At the beginning of this 

 period the somatic plate has a somewhat triangular contour, being 

 widest at the posterior pole of the embryo, and diminishing in breadth 

 from behind forward (Fig. 41). Its chief axis coincides with the 

 mid-dorsal line of the embryo, and it is connected in front with the 

 apical plate by a narrow band of small cells, which I have called 

 the dorsal cord of ectoderm. The latter, which fills the dorsal inter- 

 ruption of the prototroch, is represented in Sipunculus by the double 

 row of ectoderm cells which Hatschek has described as underlying 

 the "Amnioncanal". 



The growth of the somatic plate during this period extends 

 chiefly laterad and ventrad by bilateral cleavage. Spindles appear 

 synchronously in corresponding cells of the two sides, their long 

 axes extending parallel to the transverse axis of the somatic plate 

 (Fig. 41, 42). The anterior margin of the plate is covered by the 

 prototroch cells, which soon after their formation flatten out and 

 slip backward over it, but a wide space is left on each side of the 

 body underneath the prototroch cells, where the somatic plate does 

 not reach the apical plate. 



Mesoderm. At the age of ten hours a single pair of mesoderm 

 cells, the teloblasts, lie beneath the surface of the somatic plate. 

 Each of them thereafter gives rise not only to a band of cells in 

 front, but also probably to the two minute cells situated close to 

 the posterior, mesial, ventral surface (Fig. 40a, 41, 42, e) of the 

 teloblast. 



I have not observed the process of formation of these small 

 cells in Phascolosoma, but the origin of similar cells in other forms 

 and their close connection with the telomesoblasts in Phascolosoma 

 make it highly probable that they are derived from the latter. The 

 mesoderm of the two sides of the body is separated by a long narrow 

 mesial endoderm cell (Fig. 40b en'drm. d). Each group of the 

 minute cells lies against the endoderm, at a distance from the 

 blastopore equal to the thickness of the posterior cells of the somatic 

 plate which cover them. This position in relation to the blastopore 

 resembles considerably that which similar cells occupy in Amphitrite 

 (Mead, 1897, textfig. 5), although in the latter they are carried 

 forward by the growth of the mesoderm bands, of which they form 

 a part. Their position in an embryo of fourteen hours is intermediate 

 between that which similar cells occupy in Nereis, Thalassema, 

 Dreisscnsia, and Unio (viz. ventral and posterior to the teloblast, i. e. 



