Book - Comparative Embryology of the Vertebrates 4

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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.

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Part IV - Histogenesis and Morphogenesis of the Organ Systems

Part IV - Histogenesis and Morphogenesis of the Organ Systems: 12. Structure and Development of the Integumentary System | 13. Structure and Development of the Digestive System | 14. Development of the Respiratory-buoyancy System | 15. The Skeletal System | 16. The Muscular System | 17. The Circulatory System | 18. The Excretory and Reproductive System | 19. The Nervous System | 20. The Development of Coelomic Cavities | 21. The Developing Endocrine Glands and Their Possible Relation to Definitive Body Formation and the Differentiation of Sex

The general procedures leading to the development of primitive embryonic body form in the chordate group of animals are:

1. Cleavage. Cleavage is the division of the egg into progressively smaller cellular units, the blaslomeres (Chap. 6).
2. Blastulation. Blastulalion results in the formation of the blastula. The blastula is composed of a cellular blastoderm in relation to a fluid-filled cavity, the blastocoel. The blastoderm of the late blastula is composed of neural, epidermal, notochordal, mesodermal, and entodermal major presumptive organ-forming areas. In the phylum Chordata, the notochordal area is the central region around which the other areas are oriented (Chap. 7). The major presumptive organ-forming areas of the late blastula exist in various degrees of differentiation (Chap. 8).
3. Gastrulation. This is the process which effects a reorientation of the presumptive organ-forming areas and brings about their axiation antero-posteriorly in relation to the notochordal axis and the future embryonic body (Chap. 9). During gastrulation the major organ-forming areas are subdivided into minor areas or fields, each field being restricted to the development of a particular organ or part. (Pp. 378, 446, 447.
4. Following gastrulation, the next step in the development of embryonic body form is tubulation and extension of the major organ-forming areas (Chap. 10).
5. As tubulation and extension of the organ-forming areas is effected, the basic or fundamental conditions of the future organ systems are established, resulting in the development of primitive body form. As the development of various vertebrate embryos is strikingly similar up to this point, the primitive embryonic body forms of all vertebrates resemble each other (Chap. II).

In the drawings presented in Part III, the following scheme for designating the major organ-forming areas existing within the three germ layers is adhered to: