4 o GENESIS OF MAN. 



in man}' respects resemble the Gastrulae of some higher animals. 

 Like them, their body consists of a simple sack with only a single 

 orifice, and even possesses the ciliary organs of locomotion. The 

 great difference lies in the nature of the cellular layers composing 

 this sack. In the Turbellaria these are the four secondary instead 

 of the two primary germinative layers. Haeckel reasons here to 

 an ancient primordial worm {Urwurm, ProtJiclmis), corresponding, 

 in all respects, with this stage of embryonic development in man 

 and the higher animals generally, and from which not only all 

 other worms, but all creatures higher than the worms, including 

 mankind, have descended. This worm-stage acquires an increased 

 interest from the circumstance that here the main trunk divides, 

 sending off the articulate branch in one direction and the mollusk 

 branch in another, leaving only the vertebrate stem. The embryo 

 assumes a certain bilateralness, the four secondary germinative 

 layers grow together at their dorsal median line, and a chorda 

 dorsalis is formed. This is the true Chordoninm-stage . The em- 

 bryo now has the closest affinities with the larval state of the As- 

 cidian, which, strangely enough, though wholly devoid of a chorda 

 in its final state, has a well-defined one in its larval state. There is 

 another creature, the Appendicularia, which possesses a chorda 

 dorsalis throughout its existence, although in all other respects it 

 is a true worm, and belongs, with the ascidians, to the Tunicata. 

 This animal is the true connecting link between the worms and 

 the vertebrates, between the ascidian and the amphioxus. The 

 hypothetical Chordonium of Haeckel, the assumed ancestor of the 

 human race at this stage, and exact representative of the embryo 

 at this period of its growth, differs scarcely at all from the Appen- 

 dicularia. It is the common ancestor of the Tunicata and the 

 Vertebrata. 



From a worm, the embryo passes directly into a vertebrate. As 

 the ascidian larvae, the appendicularia, and the amphioxus are 

 separated only by the smallest differences, although the two former 

 are clearly worms, while the latter is clearly a vertebrate, so the 

 corresponding transition stages in the embryo are distinguished 

 only by almost imperceptible shades. 



The future man is now a vertebrate, but without distinct ver- 

 tebrae. He is wholly without brain or cranial enlargement, v/ith- 

 out a regular heart, without mouth, without limbs. He now be- 



