p 



ANNELIDA 323 



not yet completely united. This has occurred, however, in 

 Fig. 152 F, which shows the embryo in profile. 



Since the layer of small cells arising from the micromeres 

 follows the growth of the germ bands, the embryo becomes 

 surrounded by a superficial cell-layer, which, according to 

 Whitman, produces the epidermis. Furthermore, the head 

 portion of the worm is said to arise from these cells, and 

 perhaps in the same way as the trunk is formed from the 

 germ bands, for the trunk alone owes its origin to these 

 bands (Whitman, Bergh). 



During the processes described certain changes, which 

 give rise to the formation of the mid-gut, have also taken 

 place in the entoderm. Even at an earlier stage certain 

 cells had separated from the upper part of the entoblasts. 

 Others succeed these, for the nuclei move out to the surface 

 of the entoblasts, surround themselves with plasma, and in 

 the form of an epithelium — the cells of which at first are 

 flat, but later become cubical — are added to the cells 

 already present. The formation of the mid-gut begins at 

 the anterior end, and progresses from there backward on the 

 ventral side with unusual rapidity. Finally the completely 

 formed mid-gut surrounds the entoblasts, which have now 

 sunk to the value of mere food-yolk. At the anterior end 

 the pharynx, which has arisen as an ectodermal invagina- 

 tion, unites with the mesenteron. A shallow depression 

 makes its appearance at a very early period in the region of 

 the ectoderm cells which are first formed (micromeres) ; in 

 later stages this comes to lie at that point where the two 

 germ bands meet (Fig. 152 m). This depression indicates 

 the future pharyngeal invagination. The latter makes its 

 appearance as a solid growth of the ectoderm, which lies in 

 the depression. Later it becomes hollow and fuses with the 

 entoderm. This (entoderm) lines a part of the proboscip, 

 whereas the remaining part of the proboscis and the pro- 

 boscis-sheath are formed of ectoderm. The anus does not 

 arise until later. 



When the embryo has developed thus far — that is, when 

 the circumcrescence is completed, and its surface is entirely 

 closed — it abandons the egg and soon afterwards the cocoon 



