GASTRULATION IN THE PIGEONS EGG. 253 



primary germ layers. Concerning the manner in which these 

 two layers arise there has been a wide difference of opinion among 

 embryologists, although a great deal of attention has been paid 

 to this question. The unsatisfactory solution of this problem is 

 due to the fact that most of the conclusions are based on incom- 



FiG. 1. Scheme for orienting the blastoderm of the pigeon's egg in cutting sec- 

 tions, a, shell ; b, blastoderm at the first appearance of the primitive streak ; c, 

 chalaza ; v, vitelline membrane ; e, wedge-shaped block of yolk containing the blas- 

 toderm which is cut out and embedded for sections. 



plete evidence. So far as I am aware, not a single observer has 

 had a complete series of normal stages of any one type from which 

 to draw his conclusions. The divergent views as to the origin of 

 the entoderm, however, can be grouped into three classes, (i) 

 A number of the older workers have maintained that it arises by 

 a process of delamination, that is, the upper cells of the seg- 

 mented disc arrange themselves into a continuous layer, consti- 

 tuting the primary ectoderm, while the deeper cells of the disc 

 form the primary entoderm. This view is not in accord with what 

 is known to occur in many other forms. (2) Others have main- 

 tained that the entoderm arises by an ingrowth of cells into the 

 segmentation cavity from a part or all of the inner edge of the 

 germ-wall. The most recent contribution supporting this view is 

 by Nowack ('02) who states that the bulk of the entoderm is 

 formed out of a mass of cells, which grows forward from the pos- 

 terior part of the germ-wall. In speaking of this forward growth 

 he says : " Es gehen namlich von der Gegend des hinteren 

 Keimwalles, als unmittelbare Fortsetzung desselben, kurze Zell- 

 strange aus, die mitten durch die Keimhohle nach vorn ziehen, 

 miteinander in Verbindung treten und eine diinne Platte von 

 verschiedener Dicke und vielen grosseren und kleineren Lochern 

 bilden. Diese Platte endet vorn und an den Seiten mit freiem, 



