390 GASTRULATION 



A. Some Definitions and Concepts 



1. Gastrulation 



According to Haeckel, the word gastrula is the name given to "the impor- 

 tant embryonic form" having "the two primary germ-layers," and the word 

 gastrulation is appHed to the process which produces the gastrula. Further- 

 more, "this ontogenetic process has a very great significance, and is the real 

 starting-point of the construction of the multicellular animal body" (1874, see 

 translation, '10, p. 123). Others such as Lankester (1875) and Hubrecht 

 ('06) did much to establish the idea that gastrulation is a process during 

 which the monolayered blastula is converted into a bilaminar or didermic 

 gastrula. Haeckel emphasized invagination or the infolding of one portion of 

 the blastula as the primitive and essential process in this conversion, while 

 Lankester proposed delamination or the mass separation of cells as the primi- 

 tive process. While it was granted that invagination was the main process of 

 gastrulation in Amphioxus, in the Vertebrata, especially in reptiles, birds, and 

 mammals, delamination was considered to be an essential process by many 

 embryologists. Some, however, maintained that the process of invagination 

 held true for all the Chordata other than the Mammalia. It may be mentioned 

 in passing that Lankester conferred the name "blastopore" upon the opening 

 into the interior of the blastoderm which results during gastrulation. The 

 words "blastopore" and "primitive mouth" soon were regarded as synonymous, 

 for in the Coelenterata, the blastopore eventually becomes the oral opening. 



The definition of the gastrula as a didermic stage, following the mono- 

 layered blastula, is a simple concept, easy to visualize, and, hence, may have 

 some pedagogical value. However, it is not in accord with the facts unearthed 

 by many careful studies relative to cell lineage and it does not agree with the 

 results obtained by the Vogt method (see Chap. 7) applied to the process 

 of gastrulation in the vertebrate group. 



One of the first to define gastrulation in a way which is more consonant 

 with the studies mentioned in the previous paragraph was Keibel ('01). He 

 defined gastrulation in the vertebrates ('01, p. 1111) as "the process by 

 which the entodermal, mesodermal and notochordal cells find their way into 

 the interior of the embryo." It is to be observed that this definition embodies 

 the concept of migration of specific, organ-forming areas. We may restate the 

 concept involved in this definition in a way which includes invertebrates as 

 well as vertebrates as follows: Gastrulation is the dynamic process during 

 which the major, presumptive organ-forming areas of the blastula (Chaps. 

 6 and 7) become rearranged and reorganized in a way which permits their 

 ready conversion into the body plan of the particular species. That is to say, 

 during the process of gastrulation, the presumptive organ-forming areas of 

 the blastula undergo axiation in terms of the body organization of the species. 

 In some animal species, this reorganization of the blastula into the structural 



