GENESIS OF MAN. 35 



even in the most different animals. We are thus led to the ques- 

 tion, whether at the commencement of development, all animals 

 were not essentially alike, and whether there does not exist for all 

 a common primordial form ? As the germ is the undeveloped 

 animal itself, so it may be reasonably stated that the simple blasto- 

 sphaere (Blasenform) is the common fundamental form out of 

 which all animals are, not only ideally, but historically devel- 

 oped." 



The next and fifth stage of embryonic development is the most 

 important of all, as it leads us directly to the consideration of 

 Haeckel's celebrated "Gastraea Theory." The ontogenetic form 

 is called the Gastrula, which differs in two important respects from 

 the Planula. Instead of a single cellular layer, as in the 'latter, 

 the Gastrula possesses two such layers, one immediately within 

 the other. These layers themselves differ from that of the Plan- 

 ula, in consisting of several rows of cells instead of one, thus form- 

 ing two distinct coats composed each of several layers of cells. 

 These two coatings are quite independent of each other, and may 

 be easily separated, which is not the case with the layers of cells 

 composing each coat. The two coats differ still further from each 

 other, in being made up of unlike cells. Those of the inner are 

 larger, softer, and darker colored than those of the outer. 



The other important distinction between the Gastrula and the 

 Planula is the possession by the former of an orifice at one point 

 on its surface, through which it receives its nourishment, and 

 excretes refuse materials. This form of the Gastrula-stage, how- 

 ever, it should be stated, cannot be identified in the higher verte- 

 brates. In man. it is represented merely by a disc-shaped thicken- 

 ing at one spot on the spherical germ, and the formation there of 

 the two primary germinative layers which extend round into a sort 

 of sack, which is the unmistakable homologue of the typical Gas- 

 trula of the lower animals. 



The process by which the embryo passes from the Planula to 

 the Gastrula state, though simple, would be somewhat tedious, and 

 the reader must be referred for the details of this transition to 

 treatises on embryology, or to Prof. Haeckel's own work. 



An extraordinary interest attaches to this stage of ontogenetic 

 development, in consequence of its carrying the embryo across the 

 line which separates the Protozoa from the Mctazoa. Haeckel in- 

 sists upon this as the primary division of the animal kingdom. 



