A STATISTICAL STUDY OF MITOSIS AND AMITOSIS. 



221 



gorged and distended cannibal. The lower two larvae, IV. and 

 V., represent stages is the development of the veliger. I have 

 not attempted to show the ova with which they are filled, nor is 

 it necessary at this time to discuss the external changes involved 

 in the transformation of a cannibal into a veliger. 



The Development of the Entoderm. 



It will prove to be an advantage if the description of the devel- 

 opment of the entoderm is begun at a stage earlier than I., Fig. 

 9. A transverse section through the earliest larva available for 

 the present purpose is shown in Fig. 1. The section is bilaterally 

 symmetrical and shows on the 

 right and left, the beginnings 

 of the external kidneys (ex.k.). 

 Beneath these rudiments, is 

 mesoderm (mes.) with indistinct 

 cell boundaries, while under 

 this layer and immediately 

 upon the yolk, is the entoderm 

 (ent.), as yet an incomplete 

 membrane composed of a few 

 spindle-shaped cells with ex- 

 tremely attenuated processes. 



Fig. 2, a section cut in plane 

 xy of stage I., Fig. 9, illustrates 

 the cellular conditions met with 

 at the beginning of cannibalism. 

 Cell boundaries in all of the 

 tissues except the external 

 kidneys (ex.k.) are obscure. 

 The ectoderm elsewhere is a 

 spongy syncitium, varying con- 

 siderably in consistency in different regions. The entoderm is 

 apparently also a syncitium, but is spongy only in the anterior 

 region A where it is impossible to define its limits. Ven- 

 trally V on the side toward the external kidney, posteriorly 

 P diammetrically opposite the cap of spongy ectoderm, and 

 dorsally D diammetrically opposite the external kidney, the; 



Fig. 1. A transverse section through 

 a young pre-cannibal, showing the exter- 

 nal kidneys [ex.k. ); beneath these the 

 mesoderm (mes.) ; and immediately upon 

 the yolk, the spindle-shaped entoderm 

 cells [ent.). 



