54 Tl\e Unity of tlie Organism 



rule is no less certain and fundamental. The developmental 

 processes in these animals are somewhat more obscure at 

 several crucial points than in the ascidians, and there has 

 been proportionately more diversity of interpretation among; 

 investigators. Nearly all, however, from H. Nitsche who 

 first pointed out the anomalies here presented, to the latest 

 students in this field, Calvet and Romer, agree to the extent 

 of recognizing that the layers of the buds in these animals 

 do not conform to the germ-layer scheme that prevails so 

 widely in ontogenesis starting from the egg. 



A good summary of the view most commonly held by 

 specialists in this field is given by Harmer. 4 "There is 

 good reason for believing that in polyzoa the polypide-bud is 

 developed entirely from ectoderm and mesoderm. This bud is 

 a two-layered vesicle, attached to the inner side of the body- 

 wall. Its inner layer is derived from the ectoderm, which at 

 first projects into the body-cavity in the form of a solid 

 knob surrounded by mesoderm-cells. A cavity appears in 

 the inner, ectodermic mass, and the upper part of the 

 vesicle so developed becomes excessively thin, forming the 

 tentacle-sheath, which is always in the condition of retrac- 

 tion. The lower part becomes thicker; its inner layer gives 

 rise to the lining of the alimentary canal, to the nervous 

 system, and to the outer epithelium of the tentacles, which 

 grow out into the tentacle sheath. The outer layer gives 

 rise to the mesodermic structures, such as the muscles, con- 

 nective tissue, and generative organs." Although this de- 

 scription may not give a very clear picture to readers un- 

 acquainted with the structure and development of the bry- 

 ozoa, the point of central importance to this discussion is 

 clear enough : The outer layer of the body wall gives rise 

 to the inner layer of the bud, and from this layer is pro- 

 duced the lining of the alimentary canal, and the entire 

 nervous system. No matter what the outer layer of the 

 body-wall and its continuation as inner layer of the undif- 



