344 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



E. G. Conklin who published in 1905 a classical contribution to chordate 

 embryology relative to cell lineage in the ascidian, Styela (Cynthia) partita. 

 This monumental work extended the principle of organ-forming, germinal 

 areas to the chordate embryo. However, the significance of the latter obser- 

 vations, relative to the chordate phylum as a whole, was not fully appreciated 

 until many years later when it was brought into prominence by the German 

 investigator, W. Vogt ('25, '29). 



Vogt began a series of studies which involved the staining of different parts 

 of the amphibian blastula with vital dyes and published his results in 1925 

 and 1929. The method employed by Vogt is as follows: 



Various parts of the late amphibian blastula are stained with such vital 

 dyes as Nile-blue sulfate, Bismarck brown, or neutral red (fig. 168A). These 

 stains color the cells but do not kill them. When a certain area of the blastula 

 is stained in this manner, its behavior during later stages of development can 

 be observed by the following procedure: After staining a particular area, the 

 embryo is observed at various later periods, and the history of the stained 

 area is noted. When the embryo reaches a condition in which body form is 

 fully established, it is killed, fixed in suitable fluids, embedded in paraffin, 

 and sectioned. Or, the embryo may be dissected after fixation in a suitable 

 fluid. The cellular area of the embryo containing the stain thus may be de- 

 tected and correlated with its original position in the blastula (cf. fig. 1 68 A, B ) . 

 This procedure then is repeated for other areas of the blastula (fig. 168C-E). 

 Vogt thus was able to mark definite areas of the late blastula, to follow their 

 migration during gastrulation, and observe their later contribution to the for- 

 mation of the embryonic body. Definite maps of the amphibian blastula in 

 relation to the future history of the respective blastular areas were in this 

 way established (fig. 169C). 



This method has been used by other investigators in the study of similar 

 phenomena in other amphibian blastulae and in the blastulae and gastrulae 

 of other chordate embryos. Consequently, the principle of presumptive, organ- 

 forming areas of the blastula has been established for all of the major chordate 

 groups other than the mammals. The latter group presents special technical 

 difficulties. However, due to the similarity of early mammalian development 

 with the development of other Chordata, it is quite safe to conclude that they 

 also possess similar, organ-forming areas in the late blastular and early gas- 

 trular stages. 



The major, presumptive, organ-forming areas of the late chordate blastula 

 are as follows (figs. 167, 169, 173, 174, 179, 180, 181): 



(1) There is an ectodermal area which forms normally the epidermal 

 layer of the skin; 



(2) also, there is an ectodermal region which contributes to the formation 

 of the neural tube and nervous system; 



