After the discovery by M. Sars^l of the analogical 

 change of generations in scyphomedusa, Steenstrup united 

 these facts to produce one biological regularity. Steen- 

 strup 's general conclusion was highly regarded by his 

 contemporaries, in particular by Baer.72 Krohn also gave 

 great significance to Chamisso's discovery and Steenstrup 's 

 ideas. He made it his task to study in detail the reproduction 

 and development of Salpa. To do this, he settled for many 

 months on the coast of Sicily, where the sea provided him the 

 necessary material. Krohn put the results of his observations 

 into a special work, 73 in which, first of all, he completely 

 confirmed Chamisso's observations, by distributing them over 

 seven species of Salpa which were, for the first time, partly 

 described by him. The comparison of the solitary and colonial 

 forms (Proles solitaria and Proles gregata, in Chamisso's 

 terminology) allowed Krohn to regulate the taxonomy of this 

 group of tunicates. He showed that salpae described under 

 different names frequently proved to be different stages of 

 the development of one and the same species (137) . Later, 

 Krohn gave the characteristics of the structure of the 

 heteromorphic generation, and also described the ovum, the 

 seminal glands, and the process of fertilization in the 

 sexual generation. 



Within the present book, the greatest significance is 

 placed on the section in Krohn' s article (111) in which he 

 discussed development of the embryo in the maternal organism. 

 After fertilization, the embryonic vesicle and the embryonic 

 spot disappear, after which the ovum enlarges in size and 

 acquires a regular spherical form. This was according to Krohn, 

 who was not completely convinced of its authenticity. Sometimes 

 the ovum is not seen like that, and in its place a round body 

 appears, raising a region of tunica of the mother and jutting 

 into its cavity in the form of papilla. This body, Krohn wrote, 

 is nothing other than the rudiment of the placenta which, by 

 deepening in the cavity of the body of the maternal organism, 

 enters in connection with two of its blood vessels. Only 

 after the formation of the placenta does the embryo begin to 

 develop, at first in the form of a very small body appearing 



71. M. Sars, "Uber die Entwickelung der Medusa aurita und Cyanea 

 caprtlata," ARCH. NATURG . , 7 (1841), pp. 9 - 34. 



72. See Chapter 23. 



73. A. Krohn, "Observations sur la generation et le develop- 

 peraent des Biphores (Salpa)," ANN. SC . NAT., 3S6r., 

 Zool., 6 (1846), pp. 110 - 131. 



585 



