For the formation of the organ of small, the anterior 

 brain forms protrusions, against which olfactory depressions 

 appear on the surface; the nasal passages develop later, 

 after the formation of the palate and upper jaw. Thus, Baer 

 wrote, "the eye is a protrusion of the brain tube through 

 the muscular layer, and the nose is a protrusion from the 

 brain to that bony region" (II 7q, p. 156 (117)). The organ 

 of taste stands by itself. "I could not recognize," Baer 

 wrote, "whether for the formation of the tongue a part of 

 the brain protruded" (II 7r, p. 157 (118)). 



Recalling that the abdominal plates of the mucous layer 

 approach each other along the length of the embryo, forming 

 an internal tube at the same time as the unlacing of the 

 embryo, Baer described the subsequent changes. In agreement 

 with Rathke, in the anteriormost part of the tube, which 

 Baer called the mouth part of the intestine, the mouth slit 

 bursts open. In the posterior, the mouth part of the 

 intestine opens by an orifice, which Wolff called the fossa 

 cardiaca, facing the yolk. Because it is not connected with 

 the heart and does not correspond with the future stomach, 

 Baer suggested calling it "the anterior entrance into the 

 intestinal canal." At the posterior end of the embryo, upon 

 initiation of unlacing a blind hole is formed; this is con- 

 verted afterwards into a tube whose end later opens. Instead 

 of Wolff's name, "lower hole" (foveola inferior), Baer suggested 

 the term "posterior entrance into the digestive canal." The 

 posterior portion of the intestine, he called the posterior- 

 communicating intestine, in agreement with Rathke. The 

 middle portion of the intestine, located between the anterior 

 and posterior entrances, at first remains flat. Upon develop- 

 ment of the mesentery, it is converted into a gutter, the 

 borders of which are formed by the intestinal plates. By 

 continuation of the unlacing, the anterior and posterior 

 portions of the intestine pull up to each other, their 

 entrances become nearer and form a general passage from the 

 intestine into the yolk sac (the intestinal umbilicus) ; at 

 the fifth day this passage draws up in a narrow canal, the 

 yolk duct (see v. 1, 5e, f, r; 6g, 7h) . 



Baer noted the historical study of digestive canal 

 development, writing that 



Wolff was the first who understood this method 

 of development and explained it in the greatest 

 394 



