102 



CLASSIFICATION, PHYLOGENY, AND EVOLUTION: 



GRADE OF ORGANIZATION 



Animal organization, especially in the simpler 

 types, is somewhat similar to that of plants. This is 

 logical because plants and animals probably were de- 

 rived from the protists. The first animals may have 

 consisted of a single, multicellular body, but one with 

 all cells much the same; in other words, they were 



cellular organisms. With time, animal structure be- 

 came increasingly more complex. Cells became 

 specialized in structure and function; cells of similar 

 structure and function became grouped into tissues, 

 tissues into organs, and finally, organs into organ 

 systems. For this reason animals can be classified 

 according to evolution of their organization as cellu- 

 lar, tissue, and organ system grades. Animals 



animal pole 



sperm entrance 



penetration path animal pole 



limit of involution ^^.-''^^^iiiii^Sv limit of pigment 



gray crescent 

 (dashed outline) 



dorsal blastopore lip 



vegetal pole 

 ventral lip of blastopore 



EGG 



4 CELL 



evenly pigmented archenteron 



8 CELL 

 completion bridge 

 ectoderm 



mesentoderm 



yolk plug 



blastocoel 



yolk plug 

 in blastopore 



SECTION OF EARLY BLASTULA 



(SAGITTAL SECTION) 

 YOLK PLUG STAGE 



16 CELL 



neural groove 



NEURULA 



mesenchyme 



neural plate 



neural groove 



midbrain 



hind brain 



subchordal rod 



/ neurenteric conal 



notochord ^^^^^1 ^^^^.^ 



blastopore 



neural plate folding . , j pineal 



J^ ^ notochord \ 



mesoderm 

 {SAGITTAL SECTION) 



mesoderm 



entoderi 

 mesoderm- 



liver diverticulum 



(CROSS SECTION) 



optic vesicle 

 stomodeum 

 solid Rathke's pouch 



oral sucker' 



NEURULA 



blastopore and posterior neuropore 

 NEURAL TUBE STAGE 



Figure 7.3 Stages in frog development from a fertilized egg to on early tadpo*e-like stage. Note 

 that this development is strictly from three germ layers. (From Malcolm Jollie, Chordate Mor- 

 phology, Reinhold Publishing Corp., New York, 1962.) 



