Gastrulation in the Pigeon's Egg. 



105 



45), and yet it is clear from measurements of sucli that the distance 

 between the inner edge of the anterior germ-wall and the entoderm 

 is growing less at the same time that the entoderm is increasing in 

 length (Table III). 



Concerning those cases in which cells (other than entoderm) and 

 yolk masses are found in the cavity, more must be said. Many 

 writers have described these elements in the chick blastoderm, and 

 as far back as 1874 Goette figured them as arising from the yolk 

 lying beneath the floor of the cavity, that is, from the central peri- 

 blast. Recently Hertwig ('03) has described them, and in speaking 

 of those that lie between the entoderm and the floor, he writes as 



TABLE III. 



* By the term space is meant the distance from the anterior limit of the ento- 

 derm to the inner edge of the exterior germ-wall. 



follows: ''Zwischen ihm und dem Dotterboden liegen in der Ur- 

 darmliohle zerstreut einzelne kugelige Embryonalzellen, darunter 

 auch grossere, dotterlialtige Kugeln, die Megasphiiren von His. 

 Letztere haben nicht den Formwert einer Zelle, da Kerne auf keine 

 Weise in ihnen sichtbar zu machen sind, wie von Gasser ('84) und 

 anderen Beobachtern festgestellt Avorden ist. Sie sind daher nur 

 vom darunter liegenden Dotter losgeloste, kugelige Ballen, die wohl 

 allmahlich zur Ernahrung der Zellen der Keimblatter aufgebraucht 

 werden. Auch im Ttnum zwischen den ])eiden Keimblattern kommen 



