REPRODUCTION AND DEVELOPMENT IN CHORDATES 



and lateral surfaces of the frog embryo normally develops into epidermis, 

 whereas ectoderm of the mid-dorsal region gives rise to the neural plate. 

 When the dorsal lip of the blastopore is transplanted beneath the ectoderm on 

 the ventral or lateral regions of a frog at the gastrula stage, the ectoderm 

 covering such a transplanted dorsal lip gives rise to a neural plate, not to 

 epidermis (Pig. 5.29). In other words, formation of a neural plate in the 

 mid-dorsal region is dependent on the localization of ectoderm cells in a 

 certain relation to those cells of the dorsal lip that move internally during 

 gastrulation. This is an example of the phenomenon of induction. 



After localization is completely established in a region, differentiation is 

 apparently independent of the influence of neighboring masses. The optic 

 cup, for example, can be completely removed from an embryo and transferred 

 to a test tube containing a nutrient solution which is changed at intervals. 

 Under such conditions differentiation of the retina will take place, although 

 the shape of the eyeball will not be normal, since the shape of an organ de- 

 pends on the mutual pressure of adjacent cell masses. Similar indications of 

 independent differentiation can be obtained by grafting parts of one embryo 

 onto other embryos in a way that provides the metabolic necessities. A 

 spectacular demonstration of the completely intrinsic nature of difTerentiation 

 has been made by separating, by the use of enzymes, the cells of certain 

 localized presumptive areas, such as the wing bud or mesonephros of the 

 chick embryo. When masses of such dissociated cells are cultured on a 

 nutrient clot in a moist chamber (cf. Fig. 2.5, p. 17), the characteristic 

 tubules of the mesonephros and cartilage of the wing bud appear in due 

 course (Fig. 5.30). 



In addition to the relations between nucleus and cytoplasm and between 

 masses of cells within the embryo, the conditions of the environment affect 

 the course of development. Development normally occurs at a certain pace; 

 anything that alters that rate produces an atypical embryo. This fact has 

 been demonstrated in different ways. If the temperature is altered, the rate 

 of development will be changed. When this is done at a time when some 

 conspicuous mass movement of cells is occurring, as during the early stages 

 of cell localization, later development may be atypical in many respects. If 

 the rate of metabolism is altered by decreasing the amount of food or oxygen, 

 similar results are obtained. One such atypical effect is the production of two 

 embryos by one zygote. Frequently such embryos are joined, but some are 

 entirely separate; these are identical twins, which are not produced by separa- 

 tion of cells during early cleavage but by some arrest of development at a later 

 stage, probably during early cell localization. Another method of altering 

 the metabolic rate is by the introduction of poisonous or unusual constituents 

 into the environment. When certain salts are added to water in which frog 

 embryos are developing, atypical localization of the optic vesicles gives rise 

 to one median eye (Fig. 5.31). 



It becomes obvious from these and numerous other experiments that devel- 

 opment proceeds normally when a closely interlocking set of circumstances 



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