472 MEAD. Vor. TX. 
can be homologized with the neuro-nephroblasts of the Oli- 
gochaetes and Hirudinea. 
In Nereis, on the other hand, the posterior end of the body 
is represented by the cells of the 
X group, —“ posterior teloblasts,’’ — 
which at first lie in contact with the 
prototroch go° from the pigment 
area (Fig. VI, ¢). These cells Wilson 
. considered as being without doubt 
the homologues of the neuro-nephro- 
blasts. 
By a shifting of the neural axis 
through go° they are carried down 
to their definitive position near the pigment cells, the space 
which they vacated being filled by the rapidly-growing latero- 
dorsal and mid-dorsal regions (/d and ce 
md, Fig.s VII and VIII). 
Thus, from the standpoint of the 
prospective 7éle of their component 
areas, the early somatic plates of the 
two forms are inverted with respect 
to each other, the anterior end of the 
plate in Amphitrite being equivalent 
to the posterior end in Nereis. Fig. vu(n). 
4. In Amphitrite the somatoblast b. 
X gives rise to the whole of the trunk ectoderm — ventral, 
lateral, and dorsal; in Nereis only to the 
ventral and mid-dorsal ectoderm, the 
latero-dorsal regions being formed from 
a?! and c2", respectively (Fig. VIII). In 
Amphitrite, it will be remembered, al- 
most the entire substance of a@** and c?" 
enters into the prototroch. 
The dorsal interruption in the proto- 
troch of Amphitrite does not persist 
after the trochophore begins to elongate ; 
but the cells on either side come to- 
gether in the middle line, and thus the 
