Johnston, Morphology of the Head. 203 



ences : the entoderm does not extend far in front of the noto- 

 chord or mouth but meets the hypophysial ectoderm at a point 

 relatively far back along the lower wall of the brain. To harm- 

 onize the facts in cyclostomes and selachians it is necessary to 

 suppose that in the cyclostomes the hypophysis has pushed 

 back further than in selachians while the entoderm and 

 mesoderm have been less well developed, and the point 

 of meeting falls beneath the infundibulum instead of at 

 the lower border of the neuropore. The criticism might 

 be made on this that the writer has arbitrarily chosen 

 the selachians, in which the entoderm is well developed, 

 in preference to the cyclostomes in which the ancient 

 stomodaeum is better developed. That the choice is not arbi- 

 trary, however, is clear from two facts. First, not only is there 

 actually a longer praeoral entoderm in selachians, but there are 

 definite paired lateral structures developed (the anterior head 

 cavities) which certainly have some segmental value whether 

 they be considered as somites, lateral plate mesoblast, or aborted 

 gill pouches. Second, the same region of the brain is related 

 to the olfactory placode as in selachians and amphibia. There 

 has actually been a greater reduction of entoderm and meso- 

 derm in cyclostomes and the brain projects farther forward in 

 relation to them. The hypophysis, however, in the absence of 

 the entoderm is able to become deeper. The writer is therefore 

 forced to the conclusion that the selachians and amphibia indi- 

 cate more reliably than the cyclostomes the anterior end of the 

 head and brain. If we fix the anterior end of the brain in 

 cyclostomes, in harmony with the selachians, at the point of con- 

 tact of the nasal pit with the brain wall it would be, according to 

 the figures of Kupffer and Koltzoff, about midway between 

 the praeoptic recess and the "lobus olfactorius impar, " or very 

 near to the anterior commissure. So also the figures of Dohrn 

 (25) and His (53). 



Since finishing the manuscript it occurs to the writer that 

 the above account might be taken to imply that the entoderm 

 extends into the region which later in the paper is called the 

 prostomium. Such of course is not the intention. The pro- 



