DAVENPORT: PROCESSES CONCERNED IN ONTOGENY. 191 
crescence. We may distinguish three sub-processes under this head, 
Entod. 
Echa. 
depending upon whether (a) the concrescence takes place along the 
free margins of layers, or (b) along their sur- 
faces, or, finally, (c) along the edge of folds. N m 
This concrescence is usually quickly fol- E e 
lowed by other processes which we will 
consider later, 
a. The concrescence of layers by their 
free edges is illustrated in the cases of the 0 
growing together of the free edges, x and 18. 
x’, of the ectoderm in the Amphioxus em- 
bryo at a stage a little later than that 
shown in Figure 23. (K. & HL, Fig. 504.) PER 
b. The concrescence of layers flatwise is 
illustrated in the formation of the verte- 0) C. 
brate mouth when the anterior end of the 
entodermal sac comes in contact with the 
ectoderm. Likewise in the formation of the 
gill slits of Vertebrates the broad bottoms na, 
of the entodermal sacs move to the ecto- 
derm. (Figure 24, I, II.) à 
c. Conerescence along the edges of two ] 
folds is perhaps the commonest of these 
three forms of concrescence, It is that by 
which in Vertebrates the neural tube is 
> 
closed (Figure 25, A, B; C); ectodermal 
Fig. 24. Part of a frontal section through an embryo of Acanthius vulgaris, of 
about the stage of Balfour's Stage I. Shows 3 stages in the formation of the gill 
slit, I, II, illustrating concrescence of layers flatwise; III, perforation, Original. 
From a preparation kindly lent me by Mr. IT, V. Neal. 
Fig. 25. Cross sections through the neural tube of embryo frogs of different 
ages, showing the concrescence of the lips of the medullary groove (A, B, C), and 
(Dy the final separation of the upper and lower layers of the fold. After II. H. 
Field, '01, Bull. Mus. Comp. Zoöl., XXI. No. 5. 
