From 1846, Krohn frequently turned to the study of the 

 phenomena of reproduction and development of different 

 invertebrates - coelenterates, worms, molluscs, crustaceans, 

 and mainly the echinoderms and tunicates. 



The development of coelenterates is described in the 

 following words — first, concerning the hydromedusa Cladonema 

 and its\ development from the polyp Stauridium^ : the polyp 

 forms buds from which the medusae are formed. Similar to 

 oceanids in these medusae, as in Ooeanidae , in the walls of 

 the stomach the sexual products develop. If mature males and 

 females are situated in separate vessels, then after a short 

 time on the bottom and walls of the latter ova can be seen, 

 covered by a closely adjacent yolk membrane. That these ova 

 are fertilized, Krohn judged by the absence of any embryonic 

 vesicle and embryonic spot (nucleus and nucleolus) . Krohn 

 mentioned I later the process of ovum division, though not 

 describing it in detail, and referring to the fact that this 

 process was observed already by Dujardin, who did not, however, 

 evaluate/ its significance. Within two days after fertilization 

 the formed larva is seen in the egg, which later on leaves the 

 ovum membrane and swims with the help of cilia. The larva 

 was characterized by a light superficial layer and included 

 a dark, probably hollow nucleus (Figure 42, A). By its structure 

 the larvae of Cladonema are similar with the young of higher 

 organisms such as planulae (Aurelia, Cyanea, Cephea) . After 

 two to five days the planula Cladonema becomes rounded, 

 situated on the bottom, loses its cilia, and is transformed 

 into a disk, not changing its internal structure. In the 

 middle of the disk appears a round, hollow hillock, which grows 

 later into a cylindrical process, composed of two layers 

 present in planulae. On the upper end of the cylinder (rudiment 

 of the polyp) four hillocks form, corresponding to the 

 external ends of the future antennae. Already at this stage 

 the first stinging capsules are seen (Figure 42, B) . Thus, 

 Krohn concluded that "Stauridium resulted from the budding 

 of the medusa Cladonema, which reproduced by ova; the young 

 developing from the ovum is again transformed into the form 



46. A. Krohn, "Uber die Brut des Cladonema radium und deren 

 Entwickelung zum Stauridium," ARCH. ANAT., PHYSIOL. 

 1853, pp. 420 - 426. 



567 



