DEVELOPMENT OF BOLOCERA 58t 
Fig. 3.—Similar section about 28 hours after fertilization. Note the 
passage of one of the cells inwards from the surface, and commencement 
of trophenchyme formation. 
Fig. 4.—Similar section about 36 hours after fertilization. 
Fig. 5.—Similar section about 48 hours after fertilization. Note the 
extremely folded and crumpled surface (see p. 580). 
Fig. 6.—Similar section about 3} days after fertilization. The folds 
have mostly straightened out leaving behind numerous groups of cells 
nipped off from their recesses and enclosed within the trophenchyme. 
The outlines of these cells disappear and the nuclei degenerate now or 
later. 
Fig. 7.—Similar section about 43 days after fertilization. Commence- 
ment of gastrulation. 
Fig. 8.—Similar section about 5} days after fertilization. 
Fig. 9.—Similar section about 7 days after fertilization. 
Fig. 10.—Similar section about 8 days after fertilization, showing 
(a) the progress of gastrulation, (b) the passage of the trophenchyme through 
the inpushing endoderm into the archenteron, and (c) the involution of 
the lips of the blastopore to form the stomodaeum. 
Fig. 11.—Longitudinal section of larva 12 days old. The shape is now 
pyriform and the cells at the aboral end are becoming elongated and 
glandular. 
Fig. 12.—Transverse section across larva 15 days old near its oral 
extremity showing the two first mesenteries—the sulco-laterals. 
Fig. 13.—Transverse section through larva 20 days old showing forma- 
tion of all the Edwardsia mesenteries, viz. (1) the sulco-laterals ; (2) the 
sulculo-laterals ; (3) the sulcar directives, and (4) the sulcular directives. 
In this specimen the last named are the smallest and were no doubt 
the latest to appear (see p. 583). The sulco-laterals are now edged by 
a down-growth of epiblast for the mesenteric filaments. 
Fig. 14.—Diagram of transverse section of attached specimen (25 days 
old) to illustrate the arrangement of the eight Edwardsia mesenteries 
which are numbered as in the previous figure, and on which the rudiments 
of muscle banners can now be made out. 
Fig. 15.—Similar transverse section of attached specimen (36 days old) 
in which a new ey A oe 5 and 6 respectively) has developed 
in each sulco-lateral and lateral Edwardsia space. Muscle banners are 
beginning to develop on their sulcular sides. The six primary hexactinian 
mesenteric pairs will consist of the sulcar directives, the sulcular directives, 
two pairs made up of two and five on either side, and two pairs made up 
of one and six on either side. 
