358 GENERAL EMBRYOLOGY 



the varied relations of the mesoderm to the other layers were 

 mentioned; in some cases the middle and inner layers arise 

 from a common rudiment, in others the middle and outer 

 layers. Among the Invertebrates there are many instances of 

 development where even the two primary layers are to be made 

 out only with considerable difficulty, as for example, in the 

 Trematodes, Cestodes, certain of the Bryozoa, etc. 



In the second place the morphogenetic value of the individual 

 layers is subject to a considerable variation. Thus in the 

 Chordata, leaving aside the Mammals, the mesenchymal con- 

 nective-tissue cells may be occasionally of "ectodermal" or 

 "endodermal/' as well as of "mesodermal" origin. The endo- 

 thelium of the heart may be "endodermal" or "mesodermal." 

 The notochord may with equal correctness be described as 

 endodermal, mesodermal, or even ectodermal, in various forms. 



Single organs like the nephridia may be composites, ecto- 

 dermal and mesodermal, or, in some cases ectodermal, in others 

 mesodermal. 



In the process of regeneration certain contradictions to the 

 germ layer theory become apparent. Organs and tissues nor- 

 mally derived during embryonic development from a certain 

 layer may, during regeneration, be produced from another 

 layer. In certain Oligochsetes new mesoderm is of ectodermal 

 origin, and the regenerated pharynx may be lined with endo- 

 dermal, rather than ectodermal cells. 



Especially in the process of budding, as it occurs in a great 

 many groups, do we find abundant exceptions to this theory. 

 In some of the Polyzoa the gut may be of ectodermal origin; 

 the nervous system and pharynx are mesodermal in some of 

 the flatworms. Analogous conditions are very common among 

 the Tunicates; here the pharynx may be endodermal or ecto- 

 dermal; the atrium and even the nervous system may be ecto- 

 dermal, mesodermal, or endodermal, in different forms where 

 in egg development the relations of these structures to the 

 germ layers are typical. 



Finally, the most important qualifications and limitations of 

 the germ layer theory grow out of the observed facts of normal 



