430 A Three Weeks' Human Embno 



it has grown around the sides of the neural tul)e shows little indication 

 of shrinkage and forms complete contact with the foldings of the tube, 

 indicating that the folds give form to the mesoderm rather than the 

 reverse. 4th. Many of the young mammalian specimens examined are 

 cut so accurately, after such perfect preservation, that no question of 

 asymmetry can be raised as might be the case with the human material 

 examined. It may be said that in all the models made by me, no matter 

 hoAv twisted or imperfect the specimen, the evidence of essential symmetry 

 is clear. 5th. As is natural to suppose, the folds arise step by step with 

 the growth and development of the tube. In human specimens they are 

 in their most typical condition during the third week, after which their 

 external creases are bridged by the gro-wth of white matter and later 

 their internal sharp lines are gradually obliterated. To realize the exist- 

 ence and probable significance one should study them when most typical 

 and when they all have approximatelv the same size as in this specimen 

 (148). 



Serial arrangement of folds. — It must not be understood that the table 

 given below represents a final conclusion as to the number of folds in the 

 brain tube. It is an attempt to bring into as nearly definite relations as 

 possible at this time, the early structures shown, with those of the adult. 

 The lobules and folds cannot be said to accurately represent the definitive 

 segments which Wilder '"' proposes for convenience in studying later 

 stages, nor do they more nearly coincide with the divisions settled upon 

 by the German committee on Anatomical terms. In fact could we start 

 without so many preconceptions from the complicated adult structure 

 and the names which have been applied to the parts our task would be 

 simpler. As it is, in the figures, as few names as possible have been used 

 and even these do not always agree with the customary usages; for 

 instance, the term Diencephal is here used for a part of the roof and 

 lateral wall, but does not, as usually understood, include the infundi- 

 bulum and the eye-stalk. Perhaps the old and indefinite term thalamus 

 would better fit the case. 



The common characteristic of all these folds (except the albicantial, 

 see below), is that each pair takes its origin in a common pocket at the 

 dorsimeson (see above Cephalic End of Brain Tube), or at the edge of 

 the membranous roof or metatela and thence radiates a greater or less 

 distance along the lateral wall of the brain. Each fold, no matter how 

 obliquely, tends in general direction across the axis of the brain tube. 



"Wilder, B. G., Reference Handbook of the Medical Sciences. 2d Ed., Vol. II, 

 1901. See also Bibliography in Wilder Quarter Century Book. 



