Gastriilatiun in the Pigeon's Egg. 73 



developing egg. In order to do this it was necessary to devise a 

 microscope-stage incubator, a photograph of which is shown in Fig. 

 IV. This apparatus is so constructed that it can be used with either 

 a binocular or compound microscope, and, in case the latter is em- 

 ployed, camera drawings can be made of the object under observa- 

 tion. 



The water in the incubator is heated by an incandescent lamp (I) 

 controlled by an electric thermoregulator (7-), which can be adjusted 

 so that any constant temperature may be maintained in the region 

 of the egg-cell (e).^ 



To study the developing egg, a hole is made in the shell and the 

 blastoderm thus exposed is covered with fresh albumin. It is then 

 placed in the egg-cell and nearly surrounded with sterile physiological 

 salt solution, and the whole dish is cohered with a thin glass plate. 

 In this moist chamber eggs develop normally at least for several 

 hours (in one case for thirty-three), and I have been al)le to study 

 them, not only during the entire period of gastrulation, but also 

 during many cleavage stages. 



III. Gastrulation. 

 A. Study of the Developing Egg. 



The individual variation in the development of pigeon eggs amounts 

 ordinarily to about two hours, although in some cases it may reach 

 as high as five hours. Owing to this variation it is difficult to set 

 exact time limits to the process of gastrulation. In general, however, 

 it may be said to occur between thirty-four and thirty-seven hours 

 after fertilization. This conclusion is based on the fact that the 

 youngest and oldest stages of gastrulation are usually found in eggs 

 taken thirty-four and thirty-seven hours respectively after fertiliza- 

 tion, and is further supported by t\ie data gathered from a study 

 of the developing egg. This does not mean that gastrulation in a 

 given egg lasts for three hours. Indeed, in all probability not oveT 

 two and a half hours elapse between the involution of the posterior 

 margin and the closing of the l)lastopore. 



*For a description of tliis incuhator, see the Biol. Bull, for May, 1908, Vol. 

 XIV, No. 6. 



