270 THEORIES ON THE ORIGIN OF VERTEBRATES 



Russian on the development of Amphioxus, published in 

 1865. This subject was followed up in two papers which 

 appeared in 1867* and 1877.- In his papers on Amphioxus 

 Kowalevsky made out the main features in the develop- 

 ment of this primitive form, and showed that the chief 

 organs were formed in essentially the same way as in 

 Vertebrates ; he described the formation of the archenteron 

 by invagination, the appearance of the medullary folds, 

 which coalesced to form the neural canal, the formation of 

 the notochord and of the gill-slits. At first he made the 

 mistake of supposing that the body-cavity arose from the 

 segmentation-cavity, but in his later paper he rightly sur- 

 mised that it was formed from the cavities of the " primitive 

 vertebrae," or mesodermal segments. The origin of the 

 notochord from the endoderm was also not made out by 

 Kowalevsky in his paper of 1867. 



Although many important details remained to be 

 discovered by later investigators, 3 Kowalevsky's work at 

 once made the development of Amphioxus the key 

 to vertebrate embryology, the typical ontogeny with which 

 all others could be compared. 



Meanwhile, in 1866 and 1871, Kowalevsky had communi- 

 cated memoirs of even greater interest, 4 in which he showed 

 that the simple Ascidians developed in an extraordinarily 

 similar way to Amphioxus and hence to Vertebrates in 

 general. His proof that Ascidians also develop on the 

 vertebrate type aroused great interest at the time, and 

 was naturally acclaimed by the evolutionists as a striking 

 piece of evidence in favour of their doctrine. The 

 systematic position of the Ascidians was at that time 

 quite uncertain ; they were grouped, as a rule, with the 

 Mollusca, and certainly no one suspected that their well- 



1 " Kntwickclungsgeschichte dcs Amphioxus lanceolatus," Mem. Acad. 

 Sci. St Petersbourg (Petrograd) (v-ii.), xi., No. 4, 1867, 17 pp., 3 pis. 



- " Weitere Studien vi. die Entwickelungsgeschichte des Amphioxus 

 l.inccolatus," Arch, fiir mikr. Anat., xiii., pp. 181-204, 1877. 



3 Particularly by Hatschek (1881) and Uoveri (1892). 



4 " Entwickelungsgeschichte der einfachen Ascidien," Mem. Acad. 

 Sci. St Petersbourg (Petrograd), (vii.), x., No. 15, 1866, 19 pp., 3 pis. 

 "Weitere Studien u. die Entwicklung der einfachen Ascidien/' Arch. f. 

 mikr. Anal., vii., pp. 101-130, 1871. 



