PLATE 8 



EXPLANATION OP FIGURES 



42 Frontal section of an embryo somewhat younger than that in figure 37. 

 The yolk masses are seen to be lying inside the secondary entoderm cells, hav- 

 ing lost the membranes of the primary entoderm. Mitochondrial mass still 

 visible with its centrosome. Yolk in the upper cell {t.y.) has been transformed 

 to a fluid mass. X 730. 



43 Sagittal section through an embryo which had not yet left the capsule. 

 Drawn to show the vitellogenous cells migrating posteriorly. X 730. 



44 Oblique transverse section through an embryo still in its capsule. Sev- 

 eral large vitellogenous cells appear between the entoderm and sub-dermal 

 muscle layer. Above are shown for comparison several cells at the tip of one of 

 the mother's vitellarial strands. X 730. 



45 Anterior end of an embryo in which the cephalic glands are differentiat- 

 ing. Embryo still in capsule. X 730. 



46 Cephalic glands at a later stage showing outlet to the exterior. Embryo 

 free in the mother's mesenchyme. X 730. 



47 Oblique frontal section of a young worm 0.30 mm. long found in sea water 

 in which mussels had been washed. Relation of young testis and ovary is shown, 

 as well as the fact that the vitellaria develop from the posterior mass of vitello- 

 genous cells (figs. 43, 44). Cellular nature of the intestine is apparent. Sub- 

 dermal and intestinal glands, and the structure of the mesenchyme are indicated. 

 X 450. 



48 Seminal vesicle of living adult containing several spermatids. X 808. 



hr., brain 



e., eye 



ent.l, primary entoderm 



ent.2, secondary entoderm 



gl.c, cephalic glands 



gl.int., intestinal glands 



gl.sub., subdermal glands 



int., intestine 



m.emb., embryonic muscle cell 



nies., mesenchyme 



mit., mitochondrial mass 



0., ovary 



ph., pharynx musculature 



sp., spermatozoon 



spt., spermatid 



S.V., vesiculum seminalis 



t., testis 



vit., vitellarium 



vit.c, vitellogenous cells 



X., vitellogenous cells in mitosis 



y., yolk 



554 



