Gastrulation in the Pigeon's Egg. 77 



Except in "rare" cases, all traces of the blastoporic bay are lost 

 by the thirty-seventh hour, and the circumference of the blastoderm 

 is again a circle. The only difference between the anterior and 

 posterior halves of the blastoderm is found in the slightly opaque 

 area lying between the areas opaca and pellucida at the middle of 

 the latter half (Fig. V, K), and even this opacity usually dis- 

 appears by the time the egg is laid, that is, by the forty-first hour. 

 Hence the surface view of a freshly laid egg will give one no indica- 

 tion of the morphological difference existing between these two 

 regions of the blastoderm. 



Throughout the period of gastrulation the entire blastoderm grows 

 less opaque — a change due to the progress made by the thinning-out 

 of the blastodisc. 



In Fig. V, L is shown one of two cases that have been observed 

 in these studies, and that are of the greatest interest. Both of 

 these blastoderms show a white opaque streak extending across the 

 dorsal lip of the blastopore from the area pellucida to the posterior 

 margin. This streak is narrow next the pellucid area, but poste- 

 riorly it becomes broader and its lateral edges are continuous with 

 the right and left margins of the dorsal lip. The streak represents 

 the line of fusion of the halves of the dorsal lip, for, as we shall see, 

 tliese halves are moving from a lateral into a median position and 



Fig. V. This figure shows a series of drawings made by the aid of camera 

 outlines from the developing egg. A-D are all from a single egg, and E-K 

 from another. L is from a free hand sketch of a blastoderm taken about 

 thirty-six hours after fertilization. The other drawings were made at the 

 following periods: A, 29 hours; B, 31 hours; C, 31 hours 45 minutes; D, 

 32 hours 25 minutes ; E, 33 hours 30 minutes ; F, 34 hours ; G, 34 hours 15 

 minutes ; E, 34 hours 45 minutes ; I, 35 hours 30 minutes ; J, 36 hours 15 

 minutes ; K, 37 hours 15 minutes. Owing to the individual differences in 

 the development and size of various blastoderms at a given time, any one 

 of the above surface views would not necessarily correspond to that of 

 another egg taken at the time indicated. The two eggs from which these 

 sketches were made were selected because they gave the appearance ordina- 

 rily met with at these times, as determined by the continuous study of 

 several eggs throughout the period covered. It is not unusual to find a 

 blastoderm, taken as much as five hours earlier than that figured in B, 

 showing a pellucid area. All the sketches are X 12. 



