I36 A. C. EYCLESHYMER AND J. M. WILSON. 



the lateral evaginations of the coelom which later unites with a 

 corresponding structure on the opposite side to form the heart. 



Fig. 5 shows an obliquely transverse section through an embryo 

 somewhat older. The anterior end of the forebrain {f.b. ) ap- 

 pears as a solid mass of elongated cells. In connection with its 

 ventral wall, the optic stalk passes obliquely outward and termi- 

 nates in the optic vesicle (o.v.). On the other side the section 

 passes through the anterior portion of the adhesive organ (a.o.') 

 which here shows its connection with the anterior end of the fore- 

 gut {g.). The foregutis almost closed offventrally through the 

 approximation of the ccelomic cavities. Between the end of the 

 brain and the optic vesicles there is a slight invagination (//.) of 

 the deep ectoderm to form the beginning of the nasal pit. The 

 character of the epithelium of the adhesive organ is now more 

 plainly shown owing to the absorption of yolk granules. It will 

 be noted that the adhesive organ is now in contact with the super- 

 ficial ectoderm (s.ec.) which extends over it as a double layer. 



Another section through an embryo a few hours older taken 

 in a sagittal plane is shown in Fig. 6. The section passes through 

 the embryo in such a direction that one of the adhesive organs 

 is cut lengthwise. The organ at this time is still in open con- 

 nection with the gut. Its antero-dorsal surface follows closely 

 the external contour of the embryo. The peripheral end of the 

 organ thus extends both posteriorly and mesially following the 

 course of least resistance. In many of the sections of this stage, 

 or slightly later, a constriction is formed around the middle of 

 each of the large organs which soon gives rise to a pair on each 

 side of the snout. 



The minute structure of the organ is very similar to that shown 

 in Fig. 5. It is composed of very high columnar epithelium that 

 presents a pseudo-stratified appearance. These columnar cells 

 are heavily laden with yolk granules which are very large and 

 dense at the bases of the cells but gradually diminish in density 

 and size toward the inner ends of the cells until these ends be- 

 come free from yolk. These yolk-free ends show that cellular 

 metabolism is here most active. The indications are, from the 

 condition of the reticulum and the stainable fine granules, that 

 they are secreting cells. Moreover the presence of small drop- 

 lets on the surface of the cells adds confirmation to this view. 



