empty vesicle. With Baer and Derbes, he considered this vesicle 

 the nucleus of the fertilized ovum and suggested that the 

 process of yolk division can begin only after the appearance 

 of this nucleus. Within three to four hours after fertiliza- 

 tion the division of the nucleus begins. Referring to Baer's 

 excellent observations, Krohn left out the description of 

 the initial processes of the division. In the conclusion of 

 the monograph he noted that the result of the yolk division 

 is the formation of cells, from which the body of the embryo 

 is formed, because the globules of division are unnoticeably 

 transformed into cells of the developing larva. The evidence 

 of this transformation is found in the contents of the cells; 

 "numerous molecules inside the last substance are nothing 

 but yolk granules, from which the division globules are 

 previously formed" (p. 29). "In the same genetic relation, 

 the nuclei of the cells are related to the vesicular nuclei 

 of the division globules" (p. 30). 



Krohn prefaced the characteristic of the fully formed 

 echinus larva with the development of the larva, suggesting 

 that knowledge about the final development must help the 

 understanding of the phenomena leading to it. The external 

 form of the pluteus, the structure of its skeleton and the 

 digestive system are represented in the drawings and are 

 described as follows. The formed larva, within eleven days 

 after the fertilization, is pear-shaped (Figure 45, F) ; it 

 possesses two pairs of limbs: the short ee and the long dd. 

 On the convex side, facing the mouth £, and situated between 

 the limbs, the anal opening c^ is present, although frequently 

 closed, which is why Johannes Miiller missed it in the larvae 

 of ophiuroids and echinus. The bilateral symmetry of the larva 

 is absolutely clear and is expressed in the pairing of limbs 

 and the calciferous skeleton carrying their branches, in the 

 situation of the mouth and the anus in the plane of symmetry. 

 Inside the larva is a cavity extending to the end of the limbs; 

 the digestive tract lies in this cavity, surrounded by loose 

 fibrous tissue. The larval surface is covered with skin; the 

 last is formed from twinkling cells, in each of which there 

 is a nucleus with nucleolus. The calciferous skeleton consists 

 of four pairs of toothed branches (Figure 45, G) , the longest 

 branches gg_ are club-shaped, the next pair is found in the 

 long limbs hh, another pair in the short limbs kk, and the 

 last pair H_ is situated across the longitudinal axis of the 

 larva. 



576 



