Gastrulation in the Pigeon's Egg. 83 



Since, in preparing sections, it is impossible to avoid entirely 

 this artifact, it is important to recognize its significance/" A failure 

 to do so might easily lead one to believe that after the completion 

 of the primary ectoderm there would be many cells within the sub- 

 germinal cavity to form a ''loose layer," and thus to attribute to 

 this latter a possible origin of the gut-entoderm (delamination 

 theory) . 



It is at about this stage of development (Fig. VII) that the initial 

 step in gastrulation occurs, but before taking up that part of the 

 description I must digress in order to make clear the probable 

 significance of the method by which the avian blastodenn thins out. 

 In this connection one naturally turns to the field of comparative 

 embryology for suggestions, and here, if I mistake not, much evidence 

 is found for an explanation of this interesting process. It will be 

 necessary, however, to call attention to some well known facts in 

 embryology, even at the risk of being somewhat tedious. 



First of all, Ave may refer back to a holoblastic egg such as that 

 of the primitive vertebrate Amphioxus. Here blastulation consists 

 merely in an epithelial arrangement of the blastomeres to form a 

 hollow sphere, and only the slightest difference in size exists between 

 the blastomeres of the vegetative and those of the animal hemi- 

 sphere — a difference, perhaps, anticipatory of a meroblastic condi- 

 tion. 



In the egg of Petromyzon, which has greater meroblastic tendencies 

 than the preceding but is still holoblastic, Hatta ('07) describes and 

 figures a thinning-out of the upper hemisphere that begins ap- 

 proximately in the region where gastrulation is soon to appear and 

 proceeds anteriorly, thus finally resulting in a one-layered condition 

 of this hemisphere. The process, however, is not finished until just 

 before the completion of gastrulation. He believes that this dif- 

 ferentiation is brought about by the deeper cells pushing in between 



"Many fixing fluids have been tried in an endeavor to overcome this arti- 

 fact, but even in the best fixed series a few cells drop down, showing that they 

 were not tightly wedged in between the upper cells. In one case I have suc- 

 ceeded in fixing an egg in an inverted position, and in this the subgerminal 

 cavity is practically free from nucleated cells. 



