subsequent embryological works . The division of the ovum 

 begins two to three hours after contact with the sperm. 

 Krohn considered that the division, at least in the first 

 stages, follows the rule of progress. The vesicular nuclei 

 of the globules of division disappear before every division 

 and then again become visible. "Instead of nuclei," Krohn 

 wrote, 



in every divided globule an absolutely peculiar 

 distribution of yolk molecules is noticed. Namely, 

 they are distributed in the form of strands, which are 

 directed from the depth, from the medial point by 

 radius in all directions to the lighter periphery of 

 the ovum and, apparently, come out from two centers of 

 irradiation. After the end of division, inside the 

 new globules the nuclei again become noticeable, then 

 these radiant figures disappear and the yolk granules 

 are found to be situated close to each other, (p. 315) 



These observations show that Krohn exactly described 

 many details of mitotic division in blastomeres. 



Embryonic development goes quickly, and a day after 

 fertilization a cercaria-like embryo with a more or less 

 developed tail is already present in the ovum membrane. The 

 body and the tail of the embryo are composed of cells which 

 are especially noticeable on the surface. The cells have a 

 polygonal form, and contain granules and nuclei in the center. 

 The axis of the tail, according to Krohn 1 s description, is 

 composed of larger rectangular cells with nuclei situated in 

 a row, one following the other, and therefore they have a 

 striated or a disjointed form (Figure 48, A, b) . 



Shortly before the final formation of the larva the tail 

 undergoes remarkable transformations. According to Krohn' s 

 observations, they amount to the following. The axis is 

 transformed into a canal, as its cellular structure gradually 

 disappears due to the destruction of the partitions between 

 the neighboring cells and the liquefaction of their contents. 

 The small cells surrounding the central tail strand are 

 transformed into longitudinal muscular fibers. On the dorsal 

 side of the larva at first appears one, and then behind it 

 another, pigmented spot of granular origin (Figure 48, A, d, e 

 and B, e, f ) . At the time of transformation this formation is 



590 



