Plate IV 



All figures are from sections' 



Fig. 39 Young blastula with abnormally large blastoccel. 



Fig. 40 Section of an early gastrulation stage, showing the flattening of the lower pole of the 

 embryo and the large entoblastic cells just before sinkmg in. Several rudimentary cells re are seen in 

 the cleavage cavity; one lies inside an endodermal cell. A polar body ph, showing mitotic activity, is 

 in the blastoccel. 



Fig. 41 Section of a young gastrula, showing a mass of entomeres within the cleavage cavity; the 

 cell marked oes is probably an oesophagoblast. At a the 12 chromosomes of an equatorial plate are 

 seen. Rudimentary cells are shown at re. 



Fig. 42 Later gastrula, showing entoblastic mass with beginning cavity en. The stippled cells in 

 this and the next two figures are cells of the larval mesenchyme. 



Fig. 43 Later stage, showing a larger enteric cavity and also the ectodermal, oesophageal invagi- 

 nation; the cells marked oes are probably CESophagoblasts. 



Fig. 44 Horizontal section of a CO2 trochophore, 31 hours old. The enteric cavity is now divided 

 by a septum into stomach st and intestine in. The prototroch and apical plate are also seen. 



Fig. 45 Egg showing submerged second maturation spindle, with chromosomes at poles. The first 

 polar body is preparing to divide. 



Fig. 46 Egg showing two resting nuclei, probably the result of the mitosis seen in last figure. The 

 mitosis of the first polar body is in anaphase. 



