202 EMBRYOLOGY. 



points of the fingers, which by the tucking under of the cloth have- 

 received a covering on their lower sides, and which project above 

 the otherwise flattened cloth, are comparable to the cephalic eleva- 

 tion. In addition we can represent the backward growth of the 

 head-fold by tucking the cloth still farther under the left fingers 

 toward the wrist. 



The hinder end of the embryo develops in the same manner as the 

 front end, only somewhat later (compare fig. 11, Plate I.). Corre- 

 sponding to the posterior marginal groove (gr), the tail-fold is so formed 

 that its ridge is directed forward and that it grows toward the head-fold. 



Where in surface-views of the blastoderm the lateral marginal 

 grooves are to be seen (fig. 121), one recognises on cross sections the 

 lateral folds (Plate I., fig. 8 sf). They grow at first directly from 

 above downwards, thus producing the lateral walls of the trunk. 

 Afterwards their margins bend somewhat toward the median plane 

 (Plate I., fig. 9s/), thereby approaching each other, and in this way 

 gradually draw together to form a tube (Plate I., fig. 10). By their 

 infolding the trunk acquires its ventral wall. 



In order to avoid misconceptions, let it be further remarked that 

 only at the beginning of their formation are head-, tail-, and lateral, 

 folds somewhat separated from one another, but that when they 

 are more developed they are merged into one another, and thus are 

 only parts of a single fold, which encloses the fundament of the embryo 

 on all sides. 



As the separate parts of this fold increase, they grow with their 

 bent margins from in front and from behind, from right and from 

 left, toward one another, and finally come near together in a small 

 territory, which corresponds approximately with the middle of the 

 surface of the embryo's belly, and is designated on the figure of the 

 cross section through this region (Plate I., fig. 10) by a ring-like line 

 (hn). Thus a small tubular body is formed (Plate I., fig. 3), which lies 

 upon the extra-embryonic area of the blastoderm and is united to it 

 by means of a hollow stalk (hn). The stalk marks the place where 

 the margins of the folds, growing toward one another from all sides, 

 have met, but a complete constricting off of the embryonic territory 

 from the extra-embryonic does not take place. 



We can also represent these conditions, if, in the previously men- 

 tioned model, we in addition fold in the cloth that covers the tips 

 of the fingers along the sides of the hand and the wrist, and then, 

 carry the circular fold thus artificially formed still farther under,, 

 even to the middle of the palm. Then the cloth forms around the 



