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PROF. E, W. MACBRIDE. 
developed. The blastula in course of the next day becomes 
a gastrula, and this stage is shown in fig. 8. In the gastrula 
represented in this figure we see that the mesenchyme is 
arranged in a circle ; it is termed the primary mesenchyme, 
because it is given off as in the Echinus embryo from the 
vegetative pole of the blastula before the invagination which 
forms the archenteron has begun. At two opposite points in 
this circle a special aggregation of mesenchyme is to be seen, 
and inside each aggregation there is already clearly to be 
made out The four-armed calcareous “star” which is the 
rudiment of the larval skeleton. Of the four arms, one is 
directed upwards towards the anterior pole of the larva ; this 
becomes, as in Echinus, the antero-lateral rod. One is 
directed backwards towards the blastopore ; this becomes the 
“body rod.” One is directed horizontally and forms the 
horizontal rod ; whilst the fourth is directed outwards into 
a very slight elevation as ectoderm. This elevation of the 
ectoderm is the first trace of the post-oral arm, and the 
ann of the star corresponding to it is the rudiment of the 
post-oral rod. This “ rod ” is double, i . e . it is represented 
by two rods parallel to each other. At the anterior pole of 
the larva is to be seen the group of specially long cells 
carrying specially powerful cilia {cil.) similar to what was 
described in the case of the larva of Echinus. The 
secondary mesenchyme, which is budded fi’om the apex 
of the archenteron and becomes the loose connective tissue of 
the larva, is well seen in the figure. The gastrula rapidly 
develops into an Echinopluteus larva by the same stages as 
those described in the case of Echinus. A concave ventral 
surface becomes defined, and to the edges of this surface the 
cilia become confined. The anterior tuft of cilia becomes 
incorporated in the anterior border of this surface, and the 
post-oral arms also arise from its border. The stomodmum 
arises as a pit on the ventral surface, and in the larva shown 
in fig. 9 this is just touching the anterior apex of the archen- 
teron, although the two cavities are not yet open into each 
other. The coelom arises, as in Echinus, as a transversely 
