62 THE EVOLUTION OF MAK. 



of the blastosphgera, and, truly prophetically, insisted upon 

 it in his classical " Entwickelungsgeschichte der Thierc" 

 (vol. L p. 223). The passage in question says : " The further 

 back we go in evolution, the more do we find a corre- 

 spondence in very different animals. This leads us to the 

 question : Are not all animals in the beginning of their 

 evolution essentially alike, and is there not a primary form 

 common to all? As the germ is the undeveloped animal 

 itself, it is not without reason that it is asserted that the 

 simple vesicular form is the common primitive form from 

 which all animals, not only ideally, but also historically, 

 develop.** This latter sentence has not only ontogenetic, 

 but also phylogenetic significance, and is all the more note- 

 worthy because the blastula of the most diverse animals, 

 and the constitution of its wall of a single ceU-stratum, was 

 not then known. And yet Baer, in spite of the extreme 

 deficiency of his empiric grounds, ventured the bold state- 

 ment ; " At their first appearance all animals are perhaps 

 alike, and are merely hollow globes." 



Next to the primaeval ancestral form of the Plansea, as 

 the fifth stage in the human pedigree, is the Gastrgea, a form 

 which arises from the Plansea. Of all ancestral forms this, 

 as we have already shown, is of pre-eminent philosophical 

 significance. Its former existence is certainly proved by the 

 very important gastrula, which is met with as a transitory 

 germ-stage in the ontogeny of the most various animals 

 (Fig. 171, liK). We found that the gastrula, in its original, 

 palingenetic form, is a globular, oval or oblong-round body, 

 with one axis which has a simple cavity with one opening 

 (at one pole of the axis). This is the primitive intestinal 

 cavity with its mouth-opening. The intestinal wall consists 



