HUMAN GERM HISTORY. 403 



THIRD MAIN DIVISION OF GERM-HISTORY. 



Man as an invertebrate Intestinal Animal. 



The human embiyo possesses the form- value of an individual of the third 

 order, an unarticulated person (a siugle metameron). The primitive in- 

 testinal cavitj is enclosed by two primary germ-layers, from the fission of 

 which four secondary germ-layers are presently formed. 



Fifth Stage : Gastrula Stage (Fig. 41, p. 213, and PI. II. Fig. 17). 



The human germ forms an Amphigastrula, consisting solely of the two 

 primary germ-layers, the skin-layer, and the intestinal layer. The cavity of 

 the primitive intestine is occupied by entoderm cells, which also plug the 

 primitive mouth. 



Sixth Stage : Chordonium Stage (Fig. 90, p. 302). 



The human germ possesses, in all essential points, the organization of a 

 worm, of which the nearest existing allied form seems to be the ascidian 

 larva. Four secondary germ-layers have developed from the two primary 

 germ-layers, and coalesce along the central line. 



FOURTH MAIN DIVISION OF GERM- HISTORY. 



Man as a true Vertebrate. 



The human embryo possesses the form-value of an articulated person, 

 or a metameric chain. The articulation principally affects the bony 

 system (pi-imitive vertebra)) and the muscle-system. The skin-sensory 

 layer is divided into the horn-plate, the medullary tube, and the primitive 

 kidneys. The skin-fibrous layer has separated into the leather-plate, the 

 primitive vertebi'se (muscle-plate and bone-plate), and the notochord. From 

 the intestinal-fibrous layer proceed the heart with the principal blood- 

 vessels, and the fleshy intestinal wall. From the intestinal-glandular layer 

 the epithelium of the intestinal tube is formed. 



Seventh Stage : Acranial Stage (Figs. 103, 107, pp. 342, 344). 



The human germ possesses, in essential points, the organization of a skull- 

 less vertebrate, similar to the developed Amphioxus. The body already 

 forms a chain of metamera, as several primitive vertebrae have become 

 distinct. The head is, however, not yet distinctly separated from the 

 trunk. The medullary tube has not yet differentiated into the brain- 

 bladders. The skull is still wanting, as are also the heart and limbs. 



