374 PROGRESS OF SCIENCE IN THE CENTURY. 



archenteron. The ectoderm gives rise to epidermis, 

 nervous system, foundations of the sense-organs and 

 so on ; the endoderm forms the lining of the future 

 mid-gut and of the various organs (such as lungs, 

 liver, and pancreas) which grow out as diverticula 

 from it, and likewise, in vertebrates, to the primi- 

 tive dorsal axis or notochord; while a third median 

 stratum of cells the mesoderm of considerable 

 definiteness above the level of the unsegmented 

 worms, gives origin chiefly to muscular and skeletal 

 tissue. 



From the work of Von Baer onwards much atten- 

 tion has been paid to these germinal layers ; in 1849 

 Huxley collated the epiblast and hypoblast of the em- 

 bryo with the two layers of cells which form the body 

 of adult polyps, like the common Hydra ; and it was 

 regarded as one of the criteria of complete homology 

 that organs similar in structure should also be homo- 

 dermic, i.e., traceable to a similar origin from the 

 germinal layers. The work of the brothers Hert- 

 wig in connection with this germ-layer-theory 

 (Keimblattertheorie) was of particular importance. 



" Gradually, however, the confidence of embryol- 

 ogists in this germ-layer-theory has been shaken 

 by the following, among other, considerations, (a) 

 What one may call the stratification of the embryo 

 is established in very different ways in different 

 types; (&) there are some cases, notably sponges, 

 where the history of the outer and inner layers can- 

 not be readily brought into line with the state of 

 affairs in the majority; (c) the mesoderm is so 

 varied in its origin (from ectoderm, from endoderm, 

 or from both) and in its expression, that the concep- 

 tion lacks even a pretence at unity; and (d) in many 

 cases the facts of development show that certain 



