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



indication of the lips of the anterior part of the blastopore, the 

 connection of which with the marginal portion has long been lost. 



Below the blastoderm proper in this specimen are a number of 

 large cells (n, figs. 10-12) mostly of rounded form, filled with large 

 granules. These are present also in the last stage, but not so 

 definitely arranged. In the blastoderm now being described they 

 become very numerous below the head-process, where they form a 

 broad axial band. A few of them are to be observed in the sub- 

 stance of the lower layer itself. In the primitive-streak region 

 they are arranged for the most iiart in a double row, one running 

 along below each lateral limit of the developing mesoblast. These 

 are evidently the bodies termed formative cells by Balfour, 

 globules of Ecker by Duval. They have been found to be derived 

 from segmentation nuclei which appear on the floor of the seg- 

 mentation cavity. It would seem probable from their arrangement 

 as above described that their special function is the conveyance of 

 nutriment directly or through the cells of the hypoblast to the 

 developing mesoblast. At a stage when the mesoblast is well 

 established they are no longer traceable. 



Blastoderms resembling that above described, were obtained 

 several times ; with slight variations in minor points all presented 

 the peculiar narrow posterior prolongation of the area pellucida 

 forming the primitive streak region. 



In a specimen incubated for sixty -six hours (plate VIII. fig. 2,, 

 the posterior prolongation was broader and less strongly marked off 

 from the rest of the area pellucida ; the head-process had a more 

 definite outline, and there was a semicircular groove which marked 

 the position of the anterior boundary of the future medullary 

 plate. On examining this blastoderm in a series of transverse 

 sections, it is found that the head-process is much larger than in 

 the preceding stage. It begins very gradually in front as a 

 proliferation of lower-layer cells ; but attains a considerable 

 thickness behind. In front there is no hypoblast distinguishable 

 in it ; but behind a hypoblastic layer becomes more evident, though 

 not sharply marked ofi" in any part. Behind, the head-process 



