34 



MANUAL OF PHYSIOLOGY. 



but they also represent the first appearance of organs or parts 

 with a specific function. The external or ectoderrnic layer is the 

 supporting, protecting, motor, and respiratory organ, while the 

 inner or endodermic layer is devoted to a primitive form of di- 

 gestion, preparing the food for assimilation, and generally pre- 

 siding over the nutrition of the body. 



Although this sac-like (Gastrula) stage is supposed to have 

 formed a step in the life history of nearly all animals, yet it forms 

 a less striking part in the development of the individuals as we 

 ascend the scale, and in the higher animals no such stage has been 

 recognized. In the Vertebrates, the germ cells derived from the 



i.-r 



Transverse section of blastoderm of chick. A. Epiblast. 

 C. Hypoblast. pr. Primitive groove. 



I?. Mesoblast. 



ovum are from an early period divided into three distinct layers, 

 owing to the layers which correspond to the Ectoderm and Endo- 

 derm of the lower organisms, forming between them a third layer 

 or Mesoblast. 



From these germinal layers all the organs and tissues of the 

 body are subsequently evolved. In embryological language the 

 three primitive layers are called epi-, meso-, and hypoblast. 



Thus it can be seen that, as we can compare the primitive uni- 

 cellular state of the lowest animals with the first egg-cell stage of 

 existence of the highest animals, so we can compare all the steps 

 of 'tissue and organ differentiation as we trace them in the embyro 

 of a mammal, with the steps of elaboration in organic and textunil 

 parts that we find in ascending the scale of animal life. 



