DAVENPORT: PROCESSES CONCERNED IN ONTOGENY. 191 



crescence. We luay distinguish three sub-processes under this head, 



Efifod.. 



Fig 24. 



depending upon whether (a) the concrescence takes place along the 

 free margins of layers, or (b) along their sur- 

 fiices, or, finally, (c) along the edge of folds. 

 This concrescence is usually quickly fol- 

 lowed by other processes which we will 

 coi>sider later, 



a. The concrescence of layei'S by their 

 free edges is illustrated in the cases of the 

 growing together of the free edges, x and 

 x', of the ectoderm in the Amphioxus em- 

 bryo at a stage a little later than that 

 shown in Figure 23. (K. & H., Fig. 504.) 



b. The concrescence of layers flatwise is 

 illustrated in the formation of the verte- 

 brate mouth when the anterior end of the 

 entodermal sac comes in contact with the 

 ectodcrai. Likewise in the formation of the 

 gill slits of Vertebrates the broad bottoms 

 of the entodermal sacs move to the ecto- 

 derm. (Figure 24, I, II.) 



c. Concrescence along the edges of two 

 folds is perhaps the commonest of these 

 three forms of concrescence. It is that by 

 which in Vertebrates the neur.al tube is 

 closed (Figure 25, A, B, C) ; ectodermal 



Fig. 24. Part of a frontal section tlirough an embryo of Acantliins vulgaris, of 

 about tlie stage of Balfour's Stage I. Shows -3 stages in tlie formation of tlie gill 

 slit, I, II, illustrating concrescence of la3'ers flatwise ; III, perforation. Original. 

 From a preparation kinflly lent me by Mr. 11. V. Neal. 



Fig. 25. Cross sections tlirough the neural tube of embryo frogs of different 

 ages, showing the concrescence of the lips of tlie medullary groove (A, B, C), and 

 (/)) the final separation of the upper and lower layers of the fold. After H. H. 

 Field, '91, Bull. Mus. Comp. ZoGl., XXI. No. 5. 



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