208 G. W. BARTELMEZ 
of the Carnegie Collection (Dandy) in figure 2a. No actual 
evagination occurs, however, as we shall see: the sulcus is merely 
an effect of the lateral migration of the ganglionic cells en masse. 
Caudally they are already more loosely arranged than they 
appear in the figures and the epithelial arrangement which 
characterizes the neural fold is disappearing. The delaminating 
mass is, however, differentiated from the surrounding mesen- 
m b 
| A 
cr. sul ~ Loe | 7 0p Sul 
Fig. 4 A projection reconstruction of the medial aspect of H87 as if cut in 
the midsagittal plane. Only nervous system and gut are represented. The cut 
surfaces are indicated by hatching. The primordia are indicated as in figure 3. 
a.i.p., anterior intestinal portal; h.b.2, second subdivision of the hindbrain; 
N.T., closed region of neural tube; or.memb., oral membrane; ph., pharynx; p.np., 
posterior neuropore; por.g., preoral gut; Rk.p., Rathke’s pouch; som.JI, these 
letters indicate position and extent of second somite; thy., thyroid evagination. 
Other abbreviations as in figure 3. X 120. 
chyme by the closer proximity of the cells to one another and by 
their deeper stain. 
At the caudal end of the anlage it is possible that some cells 
are migrating out into the mesenchyme, but the plane of section 
is oblique, making the microscopic picture difficult to interpret. 
In the next embryo, ‘Du Ga’ of Eternod, we find a great in- 
crease in the cephalic mesenchyme; the head and especially the 
first two visceral arches are taking form. The neural folds have 
