REPRODUCTION 1 23 



Respiration and excretion may be carried on at all surfaces, without 

 specialized organs, as in Coelenterates. 



Ectoderm and endoderm provide for the bare necessities of animal 

 life. The addition of a mesoderm provides for some of the luxuries, 

 especially that of being large and powerful. Swift movement of massive 

 bodies demands powerful motors. Muscles are necessarily bulky. To 

 meet the metabolic requirements of cells which are deep within some bulky 

 mass of tissue such as a muscle, a circulatory system is necessary. Blood 

 carries to deep-seated tissues the food and oxygen which are otherwise 

 not directly accessible to them, and removes wastes from them. In order 

 that the blood stream of a large animal may be properly supplied with 

 oxygen and freed of waste materials, it must be related to efficiently 

 specialized respiratory organs — gills or lungs, and excretory organs, 

 kidneys. With increase in bulk and in structural complexity provision 

 for mechanical support becomes necessary. There must be tensile 

 connective tissues and rigid supporting structures — skeleton. 



Mesoderm is the main source, directly or indirectly, of these organs 

 which are of accessory nature. There are, admittedly, small animals 

 which possess a mesoderm, but there are no very large animals which do 

 not have mesodermal structures. 



Mesoderm produces directly the striated musculature of the body wall 

 and the essential tubules of the kidney. Indirectly, by way of the 

 mesenchyme (receiving some very minor contributions from endoderm 

 and ectoderm), arise circulatory organs, connective tissues, and skeleton. 

 Respiratory organs are essentially vascular and insofar derived from 

 mesenchyme, but aeration of the respiratory blood demands the inter- 

 mediation of a layer which is directly or indirectly in relation to the 

 external medium. The endoderm plays this role for internal gills and 

 lungs, the ectoderm for external gills. It is particularly the pharyngeal 

 endoderm which is the seat of development of specialized respiratory 

 organs, gills and lungs, in vertebrates. In general, therefore, the verte- 

 brate endoderm is concerned with the business of dealing with those 

 materials, food and oxygen, which the animal must receive from the 

 environment. 



There may seem to be some inconsistency between the fact that the 

 notochord, that most fundamental structure of the vertebrate skeleton, 

 is derived wholly or in part from endoderm and the fact that cartilage and 

 bone, the definitive skeletal materials of vertebrates, are derived from 

 mesenchyme. But the notochord is very ancient. Its presence and 

 condition in such animals as Amphioxus and the urochordates makes that 

 certain. While the notochord is not derived from already-formed meso- 

 derm, it develops, not only in Amphioxus but in all vertebrates, simul- 

 taneously with the mesoderm and the two are very closely similar in their 



