this view's favor — on the manner of appearance of 

 the digestive tract — is the fact that it also 

 indicates that the wall of the canal is absolutely 

 equivalent to the skin in thickness and structure. 

 (p. 18) 



Krohn's discovery of the nature of the formation of the 

 larval intestine of echinus undoubtedly possesses outstanding 

 significance. This, unquestionably, is the first description 

 of the invaginated gastrulation phenomenon. Its study sub- 

 sequently played a great role in the progress of comparative 

 embryology. Comparative analysis of methods of separation of 

 the endoderm is included in the basis of Kovalevsky f s evolu- 

 tionary concept about the formation of the embryonic layers, 

 and also in the basis of Haeckel's gastrula theory. Krohn's 

 priority in this question was not definitively underlined. 

 Mechnikov, in the article just cited, wrote the following. 



Derbes (1847) described the formation of the larva of 

 Echinus esoulentus and mentioned a stage in the form of 

 a double sac with skin layer in which the caecum opened 

 to the outside. Twenty-five years later, Haeckel gave 

 this the name gastrula, which was accepted by scientists 

 all over the world. Derbes thought that the opening 

 of the rudiment of the intestine was the mouth, but 

 August Krohn (1849) showed that it corresponded to the 

 anus of the pluteus. He described the process of 

 protrusion itself. 



Krohn referred to Derbes' observations: "The reader can form 

 an excellent concept of the gradual formation of the digestive 

 tract just described by looking at Figures 13 and 14 in 

 Derbes' article; however, the author apparently did not pay 

 attention to the process of formation itself" (p. 19). The 

 following is written by Derbes: "The spherical form of the 

 larva is changed by pressure at one point of the surface. 

 Gradually this pressure becomes more pronounced, and its 

 center is penetrated by an opening which leads to a rudiment 

 of the intestinal cavity. Beginning from this moment, the 

 movement of this opening is always directed forward and, later, 

 upwards. . . that is, the mouth looks towards the zenith" 

 (pp. 91 - 92). It is clear that Derbes did not put the deepening 

 on the surface of the spherical larva in genetic relation with 

 the formed intestine. According to his opinion, the opening 

 formed in the center of the deepening united in an unknown way 



581 



