ORGAN-FORMING AREAS 343 



173). An upper, epiblast layer is present, composed of presumptive ecto- 

 dermal, neural, notochordal, and mesodermal cells, while a hypoblast layer 

 of entodermal potency lies below. Between these two layers the blastocoelic 

 space is located. However, in the chick blastoderm, in addition to the formative 

 cells, a peripheral area of auxiliary or irophohlasi (periblast) tissue is present. 



B. History of the Concept of Specific, Organ-forming .\reas 



The idea that the mature egg or the early developing embryo possesses 

 certain definite areas having different qualities, each of which contributes 

 to the formation of a particular organic structure or of several structures, 

 finds its roots in the writings of Karl Frnst von Baer, 182S-1837. \'on Baer's 

 comparative thinking and comprehensive insight into embryology and its proc- 

 esses established the foundation for many of the results and conclusions that 

 have been achieved in this field during the past one hundred years. 



Some forty years later, in 1 874. Wilhclm His in his book. I nsere Korperform, 

 definitely put forth the organ-forming concept relative \o the germ layers of 

 the chick, staling that "the germ-disc contains the organ-germs spread out 

 in a flat plate," and he called this the principle of the organ-forming germ- 

 regions (Wilson, *25, p. 1041 ). Ray l.ankcster. in 1877, advanced views 

 supporting an early segregation from the fertilized egg of 'already formed 

 and individualized" substances, as did ('. O. Whitman ( 1878) in his classical 

 work on the leech, Clepsine. In this work. Whitman concludes that there is 

 definite evidence in favor of the preformation of organ-forming stuffs within 

 the egg. Other workers in embryology, such as Rabl, Van Beneden. etc., began 

 to formulate similar views (Wilson, '25, pp. 1041-1042). 



The ideology embodied within the statement of Ra\ 1 ankesier referred 

 to above was the incentive for considerable research in that branch of em- 

 bryological investigatiim known as "cell lineage." To quote more fully from 

 Lankester"s statement in this eonnecluni. p. 410: 



1 hough the substance of a cell may appear homogeneous under the most powerful 

 mieroseopc. excepting for the fine granular matter suspended in it. it is quite pos- 

 sible, indeed certain, that it may contain, already formed and individualized, various 

 kinds ot physiological molecules. Ihe visible process of segregation is only the 

 sequel of a differentiation already established, and not visible. 



The studies on cell lineage in many invertebrate forms, such as that of 

 Whitman ( 1878 ) on Clepsine, of Wilson ( 1892 ) on Nereis, of Boveri (1892) 

 and zur Strasscn (1896; fig. 163B) on Ascaris, or the work of Horstadius 

 ('28, '37; fig. 163A) on the sea urchin, serve to emphasize more forcefully 

 the implications of this statement. In these studies the developmental pro- 

 spective fates of the various early cleavage blastomeres were carefully observed 

 and followed. 



Much of the earlier work on cell lineage was devoted to invertebrate forms. 

 One of the first students to study the matter in the phylum Chordata was 



