580 ON THE EARLY STAGES IN THE DEVELOPMENT OF THE EMU, 



the so-called * head-process ' (Kopffortsatz) of the primitive streak, 

 the significance of which will be discussed later on. It begins 

 very gradually in front and passes behind without interruption 

 into the primitive streak. In this ' head process ' as well as in the 

 region of the primitive streak (plate XII. fig. 9) the lower layer 

 presents below, here and there, a flattened cell. These flattened 

 cells are very far at this stage from forming a complete layer in 

 this part of the blastoderm ; but there can be no doubt that they 

 are the first-formed elements of the definitive hypoblast produced 

 by modification of some of the lower-layer cells. In the middle 

 of the primitive-streak region those cells are more numerous, and 

 for a short distance form a complete layer ; but not even there are 

 theyseparable from the rest of the lower layer except by their shape. 

 The two lateral halves of the primitive streak are completely 

 coalescent, there being at no point any indication of the "suture 

 or of the canals which are to be seen at a later stage. The primi- 

 tive streak is continued backwards for some little distance over 

 the area opaca as a thickening of the epiblast. It is noteworthy 

 that there is no appearance of a ' sickle', which if it existed as in 

 the chick and some other carinate birds, would be recognisable in 

 this series of sections. 



In a specimen which had been incubated for seventy hours, in which 

 the entire blastoderm was about two centimetres in diameter, the 

 area pellucida (plate VIII, fig. 1), four or five millimetres in length, 

 had attained a shape very unlike that which it presents at this 

 period in the fowl. It consisted, as in the previous stage, of two 



parts an anterior part, which was nearly circular, and a 



posterior part, which had the form of a narrow prolongation of 

 the anterior part. This posterior prolongation is now of con- 

 siderable length. On its surface, and extending forwards towards 

 the centre of the rounded part of the area pellucida, was the 

 primitive streak, having running along its axis a well-developed 

 primitive groove, which became lost behind on the inner margin 

 of the area opaca. The primitive streak ended in front in a not 



