354 GEORGE A, BATES 



even a syncytium, and the location of the Une of separation would 

 be impossible. 



It is obvious that if there be any contribution to the duct from 

 the ectoderm it must take place at the point where the two layers 

 come into contact, which is at the growing tip of the former. In 

 order to cover this phase of the problem several series of cross sec- 

 tions have been drawn. The first series is from an embryo of 5 

 mm. and shows the terminal cell at figure 10. The section has cut 

 through the cell at its extreme edge and left only a fragment of the 

 cytoplasm adhering to the ectoderm. The fragment is composed 

 of clear protoplasm and is quite thick, as may be seen from the 

 fact that the next section has not cut entirely through it. This 

 clear cytoplasm seems to be a constant feature in the structure of 

 these cells and, in many instances, appears to be extra material 

 forming the medium of attachment between the tip of the duct 

 and the ectoderm. The next section has cut across the cell, in- 

 cluding the nucleus. 



Two features are especially striking in this series; first, the pe- 

 culiar mode of attachment of the anlagal cells to the ectoderm by 

 means of the clear cytoplasm above-mentioned, as observed in 

 sections 10, 12, and 13, and, second, the thinning out of the ecto- 

 derm. In figures 10, 11 and 12 the contact of the anlage seems to 

 have exerted no possible influence, as there is considerable space 

 between the two layers. 



As the sections begin to involve the larger portion of the anlage 

 where several cells are included in each cross section of the duct, 

 this space appears and disappears showing how loose the attach- 

 ment between the layers is, except at the outer edge of the anlage 

 where the clear cytoplasm seems to act as the direct medium of 

 union. Particular attention is called to the distinctness with 

 which, the line of separation between the ectoderm and anlage is 

 shown in the. form of the limiting membrane. If the material had 

 been prepared by any method which made this line invisible, 

 the distinction between the two layers would have been well 

 nigh impossible. It would have presented the appearance of a 

 syncytium with the consequent difficulty in distinguishing to 

 which layer the cells belonged. In order to be satisfied of the 



