Tne Cnordate Blastula and Its Si^niiicance 



A. Introduction 



1. Blastulae without auxiliary tissue 



2. Blastulae with auxiliary or trophoblast tissue 



3. Comparison of the two main blastular types 



B. History of the concept of specific, organ-forming areas 



C. Theory of epigenesis and the germ-layer concept of development 



D. Introduction of the words ectoderm, mesoderm, endoderm 



E. Importance of the blastular stage in Haeckel's theory of "The Biogenetic Law of 

 Embryonic Recapitulation" 



F. Importance of the blastular stage m embryonic development 



G. Description of the various types of chordate blastulae with an outline of their organ- 

 forming areas 



1. Protochordate blastula 



2. Amphibian blastula 



3. Mature blastula in birds 



4. Primary and secondary reptilian blastulae 



5. Formation of the late mammalian blastocyst (blastula) 



a. Prototherian mammal, Echidna 



b. Metatherian mammal, Didelphys 



c. Eutherian mammals 



6. Blastulae of teleost and elasmobranch fishes 



7. Blastulae of gymnophionan amphibia 



A. Introduction 



In the previous chapter it was observed that two main types of blastulae 

 are formed in the chordate group: 



(1) those blastulae without accessory or trophoblast tissue, e.g., Amphi- 

 oxits, frog, etc. and 



(2) those possessing such auxiliary tissue, e.g., elasmobranch and teleost 

 fishes, reptiles, birds, and mammals. 



1. Blastulae Without Auxiliary Tissue 



The blastulae which do not have the auxiliary tissues are rounded affairs 

 composed of a layer of blastomeres surrounding a blastocoelic cavity (figs. 



340 



