240 haeckel's anthropogenie. 



oxus, showing that the perforated anterior portion of the alimentary 

 canal enclosed in an setrium is common to both. The resemblance, 

 however, ends here in most adult ascidia, except that the hypo- 

 branchial furrow is also represented. It is before their retrograde 

 metamorphosis begins that the likeness is most marked. The dis- 

 covery of the embryology of amphioxus in 1866, by Kowalevsky, 

 furnished the mode of passage from the invertebrate to the vertebrate 

 group, which seemed so impossible from the point of view of the 

 " type-structure," according to Cuvier and Baer. The egg of amphi- 

 oxus, after passing through the stages of morula and blastosphere, 

 becomes by invagination a gastrula, with a ciliated exoderm. This 

 stage has since been shown to exist in all the different sub-kingdoms, 

 and this fact has caused Haeckel to propose his "gastrsea theory," 

 which is, that all animals, except protozoa, are descended from an 

 ancestral form (gastrsea), which consisted of a primitive intestine 

 surrounded by a two-layered body-wall. In amphioxus the gastrula 

 becomes flattened, a primitive groove appears and is transformed into 

 the medullary tube, which remains open anteriorly — the two primary 

 layers of the germ split into the four secondary, and the chorda is 

 d.eveloped in the upper of the two middle layers. The gastrula 

 mouth becomes the anus, and a new mouth is formed ; metamera begin 

 to appear from before backwards ; and a fold growing over the gill 

 fissures, and coalescing with that of the other side, constitutes the 

 atrial cavity. 



If we compare the above with the development of an ascidian 

 (phallusia), we shall find agreement to the minutest particulars, ex- 

 cepting in the fact that the chorda does not extend so far forwards 

 between the medullary tube, which has a well marked anterior vesi- 

 cular expansion, and the intestinal canal. When it reaches this stage 

 of development it bursts the egg-case and swims about freely as the 

 ascidian tadpole-like larva. After some time it becomes fixed, and 

 as retrogression advances, all likeness to amphioxus disappears ; the 

 medullary tube shrinks up, and nothing is left but the supra oeso- 

 phageal ganglion ; the chorda disappears with the tail, and a cellu- 

 lose sac is secreted by the epidermis, which heightens the dissimilarity. 

 It is thus among animals allied to tunicata that we must look for 

 the bridge which allows us to pass from the invertebrate to the verte- 

 brate group ; and the latest invertebrate ancestors of man must have 

 been closely connected with that group. 



