plates split into two layers, the external and internal. 

 Wolff described the corresponding changes completely and 

 correctly but not perfectly clearly. Thus misunderstandings 

 later occurred, caused mainly by Wolff's term "intestinal 

 fissure." Actually this fissure represents the slit between 

 the layers of the mesentery which later close up. Even Pander 

 understood Wolff incorrectly, and did not know the fact. 7 The 

 results of Baer's specific investigations led him to conclude 

 that Wolff's description was unclear only because he did not 

 distinguish the mucous from the vascular layer. If Pander's 

 discovery of the division of the embryonic layers was applied 

 to Wolff's description, then everything would have become 

 clear. 



After the division of the abdominal plates into two 

 layers, it is possible to see that the internal layer in its 

 turn is composed of two layers — an external vascular, and an 

 internal mucous (endoderm and visceral layer of the mesoderm) . 

 Subsequent to that in the external layer, two layers also 

 become prominent: the serous sheet and the "generating 

 tissue," forming, in Baer's opinion, the fibrous tissue, 

 bones, muscles, nerves, and walls of the body (the ectoderm 

 and the parietal layer of the mesoderm) . This external layer 

 of the abdominal plates, together with the spinal plates, 

 form the animal part of the body. The internal layer of the 

 abdominal plates forms the beginning of the vegetative part. 

 Protrusion of the internal layer toward the yolk along the 

 middle line where the vascular is not separated from the mucous 

 layer, produce a longitudinal gutter, whose borders bend from 

 the sides under the body of the embryo. These lateral plates 

 Wolff had named the "false amnion," and the gutter itself 

 "the mouth of the false amnion." Because the lateral plates 

 join together anteriorly and move into the head cap, and 

 posteriorly they join in the caudal cap, these lateral folds 

 Baer called the lateral caps, or folds, which represent a 

 part of the united common fold. 



The parts of the internal layer of the abdominal plates, 

 descending vertically to the yolk, form the layers of the 

 mesentery which comes together shortly below (Fig. 27, 5). 



Baer refers to this on page 22 of Pander's German 

 dissertation. 



320 



