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 iinder 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. ^ sf), 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 

 ovlj 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-hke 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 o£f 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- 



