442 THE EVOLUTION OF MAN. 



that of all Invertebrates, the Mantle-animals, and among 

 the latter the Ascidians, are the nearest blood-relations to 

 the primaeval parent-form of Vertebrates. An extinct species 

 of the very varied Worm tribe must be assumed as the 

 common parent-form of both groups. 



In order fully to appreciate this extraordinarily im- 

 portant circumstance, and especially in order to gain a secure 

 basis for the desired genealogical tree of Vertebrates, it is in- 

 dispensable to note minutely the germ-history of these two 

 remarkable animals, and to compare the individual develop- 

 ment of the Amphioxus stage by stage with that of the 

 Ascidian. (Cf. Plate X., and p. 436.) We will begin with 

 the Ontogeny of the Amphioxus (Plate X. Figs. 7-12). 

 Kowalevsky had already spent several months in Naples 

 with the express intention of studying the wholly unknown 

 germ-history of the Amphioxus, before he succeeded in 

 observing the mature eggs in the first stages of development. 

 He says that the Lancelet begins to deposit its sexual products 

 In the month of May, in the warm evening hours, between 

 seven and eight o'clock. 119 He noticed that at this time, 

 the male animal first ejected a whitish fluid, the sperm, and 

 that, somewhat later, the female, attracted by the sperm, 

 also deposited its eggs in the water. 



According to other observers the deposit of the sexual 

 products is said to take place through the gill-pore (porus 

 branchialis). The eggs are simple roundish cells. They 

 have a diameter of only ^ of a millimetre, are, therefore, 

 only half as large as mammalian eggs, and offer no special 

 peculiarities (Plate X. Fig. 7). The active elementary 

 bodies of the male seed, the pin-shaped " seed-animals," or 

 sperm-cells, all resemble those of most other animals. (Cf. 



