GASTRULATION AND EARLIEST DEVELOPMENT 163 



species, the gastrulation in yoli^-Jaden eggs may be easily 

 traced back to the simple process in Amphioxus. 



A peculiar feature of the gastrulation in Vertebrates which 

 has greatly contributed to the prevailing confusion of opinions 

 is the eccentric way in which the border of the blastopore 

 contracts to the final narrow opening that passes into 

 the neurenteric canal. We shall revert to this. Firstly we 

 must consider the questions: which stage represents the 

 gastrula, what must we understand by the gastrulation 

 and what must we call ento- and ectoderm? If, to answer 

 these questions, we look to the Invertebrates for a comparison, 

 especially to the Protostomia, and keep in mind the consider- 

 ations given above, our conclusion must be: the gastrulation 

 is the sinking away from the surface of part of the cells 

 and the contraction of the blastopore-border over them. 

 The gastrula, as a consequence, is the stage where 

 the blastopore-border has contracted to a very narrow, often 

 slit-like, opening that passes into the neurenteric canal in 

 the same way as in Protostomia it passes into the corres- 

 ponding cardiac pore, as I (1917b, p. 1267) have proposed 

 to call the passage from the ectodermal stomodaeum into the 

 entodermal gut. The ectoderm then, is what lies at the 

 surface in this stage and the primary entoderm what lies 

 in the interior, lining the archenteric cavity. 



Different views. — Several authors have come to other 

 conclusions by paying more attention to the histological 

 difference than to the topographical relation of the two 

 primary germ-layers. The former, just as in Invertebrates, 

 may become evident during the cleavage, long before 

 gastrulation sets in. In the Amphibian egg e.g. we can 

 distinguish the future ecto- and entoderm by the colour and 

 the size of the cells in early cleavage-stages, though no 

 sharp boundary between the two can be traced as yet. For 

 Amphioxus the same holds good, though here the difference 

 between the cells of both areas is less conspicuous and the 

 transition from the one into the other equally gradual. 



This circumstance, together with the above mentioned 

 peculiarity of the eccentric blastopore closure by which 

 the gastrulation and the gastrula distinguish themselves 

 from those of Invertebrates, has induced some authors to 

 put forward the view that already in what we call the blas- 

 tula-stage and during the cleavage of the egg the differen- 

 tiation of ecto- and entoderm is completed and that, if we 



