690 LIFE : OUTLINES OF GENERAL BIOLOGY 



dermic. Ordinary teeth start from an ectodermic ingrowth (the 

 enamel-germ) which becomes closely associated with a mesodermic 

 papilla that forms such parts as the ivory and the pulp. In the 

 development of the mouth and the gill-pouches there is a co-opera- 

 tion of ectoderm and endoderm. In the puzzling suprarenal body 

 there is a medullary portion developed in connection with the 

 sympathetic nervous system, and therefore ectodermic, and a cortical 

 portion derived from part of the embryonic excretory system, and 

 therefore mesodermic. 



Fig. I02. 



Excretory Cells in Various Types of Animals, i, A tuft of solenocytes leading 

 into an excretory tubule of a marine worm. The nuclei are marked by 

 dark circles. After Goodrich. 2, The whole excretory tubule. 3, The inner 

 end of a nephridium in Amphioxus. After Boveri. 4, A group of soleno- 

 cytes from a nephridium of Amphioxus. After Goodrich. 



TISSUES. — A tissue is an aggregate of similar cells with predomi- 

 nantly similar function or functions. Thus pieces of flesh (muscle), 

 brain, fat, liver, and gristle may be mentioned as instances of animal 

 tissue. Among plants, tissue may be illustrated by the transparent 

 skin or epidermis, or by the green middle region of a leaf, by bark, 

 wood, and pith, by the substance of a root or tuber, and so forth. 



If we compare a body to a city, and the cells to the individual 

 buildings, then a tissue may be likened to a street of similar houses 

 or shops. As to an organ, it is more like a unified "quarter" of a 

 city, such as the municipal buildings, the transport or the cleansing 

 department, the industrial quarter, the ]X)wcr stations, and so on. 

 The analogy is not to be pursued far, but it is useful for a first 

 glance. 



In the history of Biology, as we have already seen, the study of 

 organs naturally preceded the study of tissues — which may be said 

 to have begun, for practical purpo.se, with Bich^t's Anatomie 



