Albert C. Eycleshymer and James Meredith Wilson 149 
A transverse section of an embryo in this stage is represented in Fig. 
49. This section is taken through the region just anterior to the ad- 
hesive organs. On either side the coelomic cavities show plainly as they 
approach the median line. The layers of the splanchnopleure are thus 
brought in such close contact above that the gut (g.) is almost closed off. 
During the time these layers are approaching they become folded back- 
ward into the ccelom on either side. In the figure, the left side is con- 
siderably in advance of the right. Through this folding there is formed 
a second closed cavity (ht.) which is the beginning of the heart. There 
is present at this time a lining layer, but its origin is uncertain. 
The section represented in Fig. 50 passes somewhat obliquely through 
another embryo in about the same stage of development. The anterior 
end of the fore brain (f. 6.) appears as a solid mass of elongated cells. 
In connection with its ventral wall, the optic stalk passes obliquely out- 
ward and terminates in the optic vesicle. On the other side the section 
passes through the anterior portion of the adhesive organ which here 
shows its connection with the anterior end of the fore gut (g.). The 
fore gut is almost closed off ventrally through the approximation of the 
coelomic cavities. Between the end of the brain and the optic vesicles, 
there is a slight invagination (n.) of the deep ectoblast to form the be- 
ginning of the nasal pits. 
The section represented in Fig. 51 is from the same series as the pre- 
ceding and three sections farther back. The section through some over- 
sight is not magnified quite so highly. The chief point of interest, as 
compared with the preceding is the rapid separation of the ccelomic 
cavities so that the gut is here widely open upon the yolk. It should also 
be noted that the cavities of the optic stalk, the fore brain and the ad- 
hesive organs are becoming apparent. 
A section of the same series is shown in Fig. 52 at the level of the 
auditory vesicles. The section shows the extension of the cavities of 
these vesicles (a. v.) In other respects the section shows nothing more 
than is shown in Fig. 46. 
The section shown in Fig. 53 is taken in a horizontal plane and shows 
practically all the structures which have been described in the series of 
transverse sections. The divisions of the brain are very clearly shown. 
Just in front of the anterior end of the fore brain is the invagination of 
the superficial ectoblast which we have previously described. On either 
side are the nasal pits (m.) with well defined lumina. Just anterior to 
these are the adhesive organs (a. 0.) made up of the coarsely granular 
hypoblastic ‘cells. Behind these are the large optic vesicles (0. v.) in 
