520 EMBEYOLOGY OF THE LOWER VERTEBRATES ch. 



A gradual change takes place in the shape of the pellucid area 

 which, up till now circular, assumes an oval or pear shape (Fig. 

 225, B) — the long axis perpendicular to the long axis of the egg- 

 shell, and the narrow end being next the observer when the broad 

 end of the egg-shell is to the left. This narrow end may be called 

 posterior from its relations to the rudiment of the embryo which 

 appears later. Together with the gradual change in the shape of the 

 pellucid area there takes place the development of the primitive 

 streak. This makes its appearance usually during the first half of 

 the first day of incubation, as a linear opacity stretching forwards 

 along the long axis of the pellucid area in its posterior third. As the 

 first day of incubation goes on the primitive streak becomes more 

 and more distinct. A longitudinal groove develops along its 

 middle — the primitive groove — while on each side of this it forms a 

 ridge, the primitive fold. 



If a number of eggs be examined during the first day of incubation 



eel. 



mes. 





Wm&- 



en 



d. 



Fig. 226. — Transverse section through primitive streak of the Fowl. 

 eet, ectoderm ; end, endoderm ; mes, mesoderm ; p.g, primitive groove. 



it will be seen that the primitive streak, as is commonly the case 

 with vestigial organs, shows extreme variability. More especially its 

 hinder end is commonly bent to one side or the other, or even 

 bifurcates into two branches. At its front end one or both halves of 

 the primitive streak swell up into a slight knob while the primitive 

 groove becomes somewhat deeper and wider. 



The primitive streak is shown by transverse section to originate 

 from a linear tract of ectoderm along which the cells are undergoing 

 rapid proliferation, as is indicated by the relatively numerous mitotic 

 nuclei. The cells budded off by the ectoderm are aggregated 

 together in a compact mass along the course of the primitive streak 

 while on each side they become loosened out and wander away into 

 the space between ectoderm and endoderm to take part in forming 

 the sheet of mesoderm. 



rest on a firm basis of knowledge of Reptilian development. At the present time 

 however our knowledge of the exact relationship of these developmental stages of 

 Birds to the corresponding stages of Reptiles is not in the present writer's opinion 

 adequate to form a trustworthy basis for their interpretation. 



