EMBRYONIC DEVELOPMENT 



2H: 



Organizers. — What induces this change in a tissue, destroying its 

 apparent independence of action, and forcing it into a single further 

 course? It is often some influence coming from other cells near it. In 

 salamander embryos, the cells which roll over the dorsal lip of the blasto- 

 pore and become the notochord and mesoderm (Fig. 180) exert such an 

 influence. It is because of them that neural folds are produced in the 

 ectoderm above the notochord. The mesoderm cells possess that power 

 of inducing nervous system even before they are invaginated into the 

 gastrula. This is beautifully shown by an experiment. If some cells 

 are removed from the dorsal rim of the blastopore, before they are invagi- 

 nated, and are inserted among the ectoderm cells of another embryo, at a 

 place where only epidermis w'ould ordi- 

 narily develop, they sink below the surface 

 and are covered over by the ectoderm. 

 From that ectoderm an additional nervous 

 system is formed, so that the embr^^o has 

 two nervous systems (Fig. 191). The 

 transplanted cells would, in their own 

 embryo, have been invaginated to form 

 mesoderm and would have induced a 

 nervous system in the ectoderm above 

 them. That same influence they exerted 

 on the strange ectoderm beneath which 

 they were planted. 



In a similar way, the eye stalk pro- 

 truding from the side of the brain (Fig. 

 184), as it approaches the outer ectoderm, 

 stimulates that layer to thicken and 

 invaginate to form the crystalline lens of 

 the eye. In some animals the ectoderm 

 forms a sort of lens without such stimulus, as when the eye stalk is cut 

 off; but the lens is seldom normal unless the optic stalk comes near it. 



Something issues from the prospective mesoderm and the eye stalk, 

 in the above examples, which causes the ectoderm to develop a certain 

 structure. This something, whatever it is, has been called an organizer. 



An important question arises, whether embryonic development is 

 conducted by a series of such organizers, produced in succession in dif- 

 ferent structures. May one organizer ensure the development of a cer- 

 tain organ, and then a different organizer arise in that organ that would 

 stimulate a third organ, and so on? Some slight indications of such 

 chains may be found, but they are not general. The eye stalk often 

 stimulates a lens, and the lens then helps to bring about the invagination 

 of the optic cup to form the retina. A few other such chains of influences 



Fig 191. — Development of 

 nervous system in response to trans- 

 planted cells. Left, neural fold of 

 salamander, Triton, developing in 

 its normal situation. Right, op- 

 posite side of same embryo, witli 

 additional neural fold produced 

 because cells from the dorsal lip of 

 the blastopore of another embrjo 

 were transplanted in that region. 

 The transplanted cells were from a 

 lighter colored species and form the 

 pale streak in the middle. {Modi- 

 fied from Spemann.) 



