The line of this union Wolff named a "junction." Wolff erred 

 only in his indication that before the formation of the 

 junction the space between the layers of the mesentery opens 

 downwards, whereas actually it is covered by a mucous layer. 

 Gradually the layers united below the mesentery begin to 

 accrete with each other. This process proceeds along the 

 long axis of the embryo, so that its variable stages can be 

 seen on different levels of the same individual. In the 

 anterior part of the body, where the digestive canal is 

 already present, the layers of the mesentery envelop it, so 

 that it is composed of the internal mucous canal and the 

 external canal formed by the vascular sheet. 



After the closure of the mesentery junction, the mucous 

 and the vascular layers protrude upwards along the middle 

 line of the body and form a gutter, the walls of which Baer 

 called "the intestinal plates" (Fig. 27, 5) and the gutter 

 itself the "intestinal gutter." It closes below but, as Baer 

 wrote, "not by means of the middle junction, but in such a 

 way that from both ends a lengthening of the already closed 

 beginning and end parts of the food canal takes place to the 

 middle" (I 5e, p. 84 (45)). By the end of the third day, 

 only the third part of the length of the digestive canal 

 remains, in the form of unclosed gutter. 



Baer warned against the simplification of the process of 

 intestinal formation which could be concluded from Wolff's 

 description and his specific data. "It is logical by itself," 

 he wrote, "that this protrusion must not represent a pure 

 mechanical process through which the layers of the rudiment 

 membrane, which were located earlier on one plane, must be 

 later made up into folds; soon this protrusion is accompanied 

 by organic growth . . ." (I 5f, p. 85 (46)). This warning is 

 important because a half century later in embryology, the 

 simple mechanical interpretation of formative processes was 

 still widely distributed. For example, gastric invagination 

 was understood as a simple protrusion of the vegetative 

 hemisphere of the blastula, and the formation of the embryonic 

 folds as a mechanical protrusion of the previously planar 

 parts of the embryonic layers. 



Baer considered the formation of the intestine, and the 

 formation of the mesentery which precedes it, and at the same 

 time the approach of the abdominal plates to each other as a 



321 



