the developed rudiment of the digestive tract with the 

 external world. Krohn pointed also to DerbSs' mistaken assump- 

 tion that this opening was the mouth. He established that the 

 opening in the area of protrusion is the anal opening, and 

 the mouth is formed in another place significantly later 

 (on the fourth day after fertilization] . 



Further observations of Krohn are concerned with the 

 changes in the larval form" as it becomes bilaterally sym- 

 metrical (Figure 45, B and C) . He described the formation of 

 limbs and the calciferous skeleton (Tigure 45, D and E) , 

 which gradually acquires a different configuration. 



In the following year, Krohn repeatedly returned to the 

 study of development of the different echinoderms and 

 published many reports on this subject, One of these reports 

 considered the development of the holothurian and echinus. 67 

 The larvae of holothurians (Holothuria tubulosa} were 

 obtained from plankton, because attempts at artificial 

 insemination proved unsuccessful. The youngest larva observed 

 by Krohn "is similar to an elongated egg (Figure 46, A); in 

 its sharp pole there is an opening leading to a sac-like 

 protrusion in the body and an ampul la- shaped canal widening at 

 the end Ik This sac is the rudiment of the digestive tract, 

 the opening doubtlessly is the anus" (p. 345). Krohn noticed 

 that the larva of Holothuria tubulosa, at this stage, is very 

 similar to the larva of Echinus lividus described earlier by 

 him (1849) . Similar to the latter, the larva of Holothuria 

 tubulosa is covered by cilia, and with their help it swims with 

 the imperforate pole forward. The surface of the body and the 

 digestive tract consist of cells which, with their nuclei, 

 become noticeable with the addition of fresh water. In the 

 body cavity, as in the larvae of Echinus lividus, fibrous 

 tissue with fusiform cells is found. Later (Figure 46, B) on 

 the abdominal side of the larva a depression appears — which 

 is the future transverse fissure of ausicularia; in this 

 fissure the mouth opening later appears. Later the body 

 acquires a kidney-shaped configuration (Figure 46, C) . The 

 digestive tract, at this time, forms the rudiments of the three 



67. A. Krohn, "Beobachtungen aus der Entwickelungsgeschichte 

 der Holothurien und Seeigel," ARCH. ANAT . , PHYSIOL. 

 (1851) , pp. 344 - 352. 



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