296 Basic Structure of Vertebrates 



the limiting surfaces of cells, proceeds at a high rate during develop- 

 ment. Cleavage increases the ratio of cell-surface to cell-volume and, 

 at the same time, increases the quantity of the essential nuclear mate- 

 rial, both absolutely and relatively to the volume of cytoplasm. 



In the blastula the embryonic material is in the form of a hollow 

 sphere (more or less modified by yolk), a form providing favorable 

 exposure of surface to the external medium, and significant in view of 

 the fact that the adult animal is built up largely by manipulation of 

 layers of cells. The blastula cavity is not permanent. 



Gastrulation transforms the one-layer blastula into a hollow two- 

 layer embryo, the gastrula, whose new cavity, produced in the process 

 of gastrulation, is the prospective digestive cavity. This archenteron 

 opens externally by the blastopore. The ectoderm, potentially pro- 

 tective and nervous, provides for relations of animal to environment ; 

 the endoderm is nutritive. Respiration and excretion may be carried 

 on at all surfaces, as in coelenterates. At the gastrula stage, therefore, 

 a minimum metazoan animal is established. 



Ectoderm and endoderm provide the bare necessities of animal life. 

 Addition of a mesoderm provides for some luxuries, especially that of 

 being large, powerful, and active. Swift movement of massive bodies 

 requires powerful motors. Muscles are necessarily bulky. The metabolic 

 necessities of cells situated deep in bulky masses of tissue can be met 

 only by a circulatory system which brings them food and oxygen and 

 removes their wastes. The circulating mediums, blood and lymph, must 

 be related to appropriately specialized organs of "intake" (digestive 

 organs, gills, or lungs) and organs of "output" (kidneys). Increase in 

 bulk and complexity calls for mechanical support. There must be 

 tensile connective tissues and rigid supporting structures — skeleton. 

 Mesoderm is the main source of all these organs, which become neces- 

 sary with increase in size of the animal. There are small animals which 

 have a mesoderm, but there are no very large animals without 

 mesoderm. 



Mesoderm produces directly the striated muscle of the body-wall 

 and the essential tubules of kidneys. Indirectly, via mesenchyme (with 

 minor contributions from ectoderm and endoderm), arise circulatory 

 organs, nonstriated muscle of the viscera, connective tissues, and 

 skeleton. The notochord is not derived from already formed meso- 

 derm but develops simultaneously with the mesoderm, and the two 

 are closely similar in manner of origin. Respiratory surfaces may be 

 provided by either ectoderm or endoderm, or by both. The pharyngeal 

 endoderm is the usual seat of origin of specialized respiratory organs, 

 internal gills and lungs. Ectoderm serves as the respiratory surface in 



