436 A Three Weeks' Human Embryo 



the myel are not strongly marked, and in other specimens it is only in 

 favorable sections that they can be seen at all. 



The Special Points Appearing from a Study of this Embryo 

 are: — 1. Both external form and internal organs show with diagram- 

 matic clearness a normal development but with individual differences 

 from other specimens of about the same age, some of these differences 

 indicating greater, some less development. It seems probable that a care- 

 ful study of such embryonic peculiarities in man and higher mammals 

 may throw light on very important questions of heredity and variation. 



2. Epithelial thickenings occur at the neuropore, olfactory region, 

 lens, gill-clefts, and about the mouth, at the summit of the limbs, the 

 thickening of the leg being continuous with that of the anal region. 



3. There are 29 myotomes, 2 being occipital, and also remnants of 3 

 other occipital myotomes. 



4. The nephric system is in a generalized condition presenting a re- 

 capitulation in one specimen of several distinct stages of development. 

 This is shown by : — An open pronephric tubule on each side, independent 

 of the Wolffian duct; each mesonephros having in its cephalic half, 8 

 rudimentary glomeruli opening by tubules into the duct; in its caudal 

 half, 11 or 12 tubules not opening into the duct, but part of them opening 

 to the coelom. The mesonephric tubules vary in structure from solid 

 masses of cells to tubules with glomerulus and Bowman's capsule. 



5. The developmental stage of the central nervous system shows with 

 definiteness the position of the neuropore and its relation to the hypo- 

 physial region. In comparison with other specimens examined this makes 

 it possible to determine the front end of the brain tube and of the body. 



6. I believe that the morphologic cephalic end of the body is as figured 

 by von Baer, in the region of the hypophysis and, furthermore, I believe 

 as a generalization, that in all stages of development the hypophysial 

 region is at the morphologic, cephalic end of the body, and consequently 

 that parts which in the exigencies of growth have gone beyond this point 

 are morphologicly caudad of it, as the eye and olfactory region. 



7. The brain tube shows both at this stage and at earlier and later 

 stages total foldings which are directly correlated with definite nerves or 

 epithelial thickenings. Other foldings have not yet been correlated with 

 definite organs. These foldings are so uniformly present in mammals, 

 birds, and selachians that they cannot be conceived of as artifacts but are 

 believed to be true morphologic features. 



