7 2 



CHILD. 



[VOL. I. 



and right. The upper pole represents approximately the 

 anterior end, and the lower pole the posterior end. In other 

 words, the median plane of the adult passes through the blasto- 

 mere D and forms an angle of 45 with the first two cleavage- 

 planes. This orientation does not agree with that given by 



FIG. i. 



FIG. 2. 



Wilson ('92) for Nereis, but I am unable to make his figures 

 agree with his orientation. Mead ('94) gives the same orienta- 

 tion for Amphitrite that I find in Arenicola. 



The enormous size of the blastomere D is interesting, as it 

 of necessity influences the whole of the cleavage. This is the 

 cell from which the first and second somatoblasts (A'' and Jlf) 

 will arise. 



In Fig. 3, the 8-cell stage, it is seen that the four so-called 

 micromeres are very large, the two dorsal ones being slightly 

 larger than the ventral pair. The position of the posterior 

 cross-furrow at this stage is considerably ventral to the poste- 

 rior pole. 



The cleavage proceeds in the typical oblique manner up to a 

 stage of fifty-eight cells. During this period, the germ-layers 

 have been separated. The ectoblasts arise as three " quartets " 

 of blastomeres, given off successively and with alternating 

 direction of spindle from the macromeres, the mesoblast (M) 

 appears as a single cell, and the entomeres are represented by 

 four cells. The large ectomere X, which is to furnish almost 

 the whole ectoderm of the trunk, is seen in Fig. 4. The cells 



