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 

 (Keimhldttertheorie) 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; (h) 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 thait certain 



