246 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
(ce, d™4, Fig. 71), one in the right and one in the left half of the 
embryo. They are now separated both from each other and from the 
anterior chorda cells. We shall see later how they are brought into 
contact with each other, in the median plane, and with the anterior 
chorda cells, 
The mesenchyme mother cells are also ten in number, but, unlike the 
chorda cells, they aro derived chiefly from the posterior quadrants. They 
are 4% 2 its deep-lying sister cell, 49, d'2, D'5 and D'S, with the 
corresponding cells in the left half of the embryo, all indicated by a flat 
tint in the Figures.’ It will be observed that the mesenchyme fundament 
is made up of cells derived from both hemispheres and all four quadrants. 
The outer of the two rows of cells encircling the endoderm fundament 
will be called the neuro-muscular ring. (Fig. 71. The cells are stippled.) 
It is interrupted at three points by mesenchyme cells of the inner ring ; 
in the middle line behind; by the small flattened cells, 075, D"5; on the 
right side, by .4*?; and on the left side, by B8" It is thus divided 
into three portions, an anterior segment of eight cells, all descended 
from the anterior quadrants, and two latero-posterior segments, each 
composed of four cells, descended from one of the posterior quadrants. 
The anterior segment is composed purely of nerve mother-cells, which will 
form a considerable portion of the medullary plate. 'The other segments 
will form the entire longitudinal musculature of the larva, as well as a 
certain portion of the nervous system in the tail region. 
In the two rings of cells just described are included all save two of 
the descendants of the cells forming the equatorial band of the 48-cell 
and later stages. These two cells are .D* and C55, situated at the 
posterior margin of the embryo (Fig. 71). They form, in my opinion, 
definitive ectoderm. 
The remaining cells of the embryo number sixty-four, all descendants 
of the ectodermal group of the 48-cell stage. They will form definitive 
ectoderm, possibly also a portion of the medullary plate. 
One again notices in this stage the striking difference in rate of di- 
vision of the cells which he meets in passing from the vegetative toward 
the animal pole, a difference which made itself apparent as early as the 
! Samassa (’94) identified the mesenchyme mother cells D78 and d712 (the cells 
8 and 9 of his Fig. 10) as nerve cells, In my preliminary paper I expressed a dif- 
ferent, opinion, stating that they were mesoderm cells. Subsequent study has 
confirmed this view, but shown that I was wrong in stating, as 1 did, that they 
would contribute to the formation of “the longitudinal musculature of the tail." 
That organ has, as I shall show, an entirely different and hitherto unsuspected 
origin. 
