EXOGASTRULATION

Exogastrulation

Holtfreter (1933) demonstrated that embryos may be made to exogastrulate, ie., the endoderm, notochord and mesoderm may be made to evaginate to the outside instead of undergoing the normal process of inward movement. For example, in the axolotl, Amblystoma, if embryos are placed in 0.35% Ringer's solution, exogastrulation occurs instead of gastrulation. The endodermal, mesoderrnal and notochordal areas lie outside and are attached to the hollow ectodermalvesicle. The exgastrulated material, therefore, never underlies the ectodermal cells, but comes to lie outside neural plate and epidermal ectoderm of the gastrula . Exogastrulation is also effected by treatment of eggs with lithium salts.

ln the exogastrulae, the presumptive neural plate and epidermal areas never develop to great extent. Neither the neural plate nor any of the sensory structures deveiop from the ectoderm .The chordamesodermal area does not move inward and makes contact with the neural ectoderm. This indicates strongly that the presumptive neural plate and epidermal areas of late blastula and early gastrula are dependent upon the normal gastrulative process for their future development. Exogastrulation also clearly separates the parts concerned with emboly from those which are moved by the forces of epiboly. Ln other words, exogastrulation results when the forces of epiboly are separated from the forces normally concerned with emboly. Normd gastrulation occurs by a precise and exact correlation of these sets of  forces.

Effects of Removal of Egg Membrane

J. Holtfreter removed the protective membranes of amphibian eggs and placed them in a slightly hypertonic solution. The eggs developed into exogastrulae instead of normal gastrulae. The prospective endoderm and chordamesodenn evaginate to form an outgrowth. The prospective endoderm differentiated into usual endodermal structures like gills, gut lining, glands the etc.The chordamesoderm differentiated into the notochord and mesodermmal somites  Thus there is a fairly normal differentiation of the mesodermal and  and endodermal derivatives The prospective ectoderm expanded into a spherical masses of  undifferentiated tissue. [t remained as primitive epithelium. No nervous  structures were developed from the ectoderm. This is because chordamesoderm fails to invaginate and thus does not make with the ectoderm to differentiate it into neural plate. Thus the development of all the sensory structures and  the nervous system is dependent upon the normalcontact between the roof of the archenteron (or chordamesoderm) and the ectoderm. Holtfreter carefully followed and compared the movements of cells during normalgastrulation and exogastrulation by employing vitai staining techniques. From these experiments, it was concluded that the grey crescent region which contains factors for normal development becomes the dorsal lip of blastopore in the early gastrula and later the roof of the archenteron (chordamesoderm). ln normal development, the chordamesoderm induces the overlying ectoderm to develop into the. nervous system.