PINEAL REGION IN MAMMALIA 93 



in certain chick embryos could be homologized with any of the 

 detached tubules or cysts shown in the human embryos will 

 require further study and confirmation. As regards Mammalia, 

 there are no signs of these postvelar tubules in any of the sheep 

 embryos here described. Heuser (9) shows several excellent 

 models of the ventricles of the pig from early stages up to 260 

 mm., but nothing comparable to what has been described in 

 human embryos occurs there and no traces can be found either 

 in rat or rabbit embryos. 



In dog embryos of 14 mm. and 17 mm. a median pouch-like 

 prolongation from the diencephalic roof is clearly seen, figures 

 31 and 32, which resembles the similar projection in the human 

 embryo in figure 13. A somewhat similar arrangement appears 

 in cat embryos of 10 to 12 mm. (fig. 33) and in older embryos 

 there is a marked projection of the diencephalic roof forward 

 over a greatly reduced velum and paraphysal arch. In the 

 cat, however, folds of plexus invade the prolongation in a manner 

 quite different from that seen in human embryos, and no 

 tubular formation can be seen. These are apparently the only 

 instances occurring in other forms which are in any way com- 

 parable to the condition just described in the human embryos. 

 It might be asked justly whether this formation was not wholly 

 pathological. It, however, occurs so constantly, although in a 

 variable degree of complexity, that it would seem to be a normal 

 phase in embryos between the ages considered here. It is also 

 suggested that the possible persistence of some of these de- 

 tached vesicles might account for some of the pathological 

 conditions found in this general region. Conditions prevailing 

 at the time when this paper was written prevented the author 

 from following out the comparative morphology of these post- 

 velar tubules as thoroughly as should have been done and more 

 extensive study of comparative forms and of older human 

 embryos will be needed to reach a satisfactory conclusion. 

 The terms 'postvelar tubules' or ' diverticuli' are used here for 

 lack of more suitable expressions and are rather unsatisfactory. 

 It is hoped, however, that ^^dth further investigation a better 

 description can be obtained. 



