REPRODUCTION AND DEVELOPMENT IN CHORDATES 



their development in the water since they have no amnion and chorion. 

 Embryonic membranes and their modifications must have been extremely 

 important in the evolution of the vertebrates, especially in the origin of 

 the mammals. 



Experimental Modification of Development 



That the orderly processes which occur during development are conditioned 

 by a number of closely interrelated factors can be experimentally demonstrated. 

 The genes, or hereditary units, carried by a zygote control its development, 

 and certain combinations of genes have been demonstrated repeatedly to 

 bring about death in animals used in experiments (p. 205). Nuclei of 

 embryonic cells are quite similar in appearance, and when they divide all are 

 found to contain the number of homologous chromosomes characteristic of 

 particular species. It has long been assumed that the nuclei of somatic cells 

 of an individual are genetically equivalent. In recent studies, nuclei from 

 cells of blastulae and early gastrulae as well as from cells of the chordamesoderm 

 and endoderm of late gastrulae were transplanted singly into enucleated 

 zygotes of the frog. Depending on the age and source of the transplanted 

 nucleus, development of the zygote varied greatly or not at all from its normal 

 pattern. It seems, therefore, that nuclei become differentiated in some way 

 during development and that their diflferences, whatever they may turn out to 

 be, are correlated with cell localization and differentiation. 



The cytoplasm of the zygote is shifted in an orderly way by streaming 

 movements after fertilization so that certain parts are located in particular 

 cells during cleavage and carried into typical positions by later movements. 

 If the zygote is subjected to strong centrifugal force so that the cytoplasm 



Fig. 5.28. Development of the frog after separation of cells and injury at the two-cell stage. 

 A, cleavage in each part after separation of the first two cells within the jelly envelopes by 

 constriction with a hair; each separated part gives rise to a complete embryo. B, a half 

 embryo at the neural-fold stage, following injury to one of the first two cells by means of a 

 hot needle. {A, after H. Spemann, 1914, Verhandlung der deulschen zoologischen Gesellschajl, 

 vol. 24; fi, redrawn from W. Roux, 1888, Archwfur pathologische Anatomie, vol. 114.) 



165 



