188 SUMMARY OF CURRENT RESEARCHES RELATING TO 



which are of interest not only in themselves, hnt as the first recorded 

 researches on the embryology of any member of the liatitne. The 

 period of incubation in the emu is three months, and the time between 

 the developmental events is therefore great when compared with the 

 development, for instance, of the chick. An average egg weighs 

 21 ounces; and measures 4 inches by dk inches. About forty are laid 

 in a summer; the male incubates the first set and is then succeeded by 

 the female who has by that time finished her laying. 



The first blastoderm studied was one of 51 hours, and measured a 

 centimetre in breadth. Tho area pellucida, which measured 3 mm. in 

 maximum diameter, presented two regions — an anterior, rounded and 

 rather broader than long, and a posterior bay, the commencement of the 

 primitive streak region. There was no trace of a primitive groove. 

 Sections showed two completed layers, separate in the anterior region, 

 confluent in the primitive streak. In front of the anterior end of the 

 primitive streak, the lower layer presents a slight thickening, the 

 rudiment of the " head-process." Flattened cells in the lower layer of 

 the head-process are the first hints of the final hypoblast. There was 

 no appearance of the " sickle " seen in the chick and other forms. 



In a blastoderm of 70 hours, the area pellucida has a shape very 

 unlike the corresponding stage in the fowl. The anterior part is 

 circular, the posterior a long narrow prolongation, bearing the primitive 

 streak which runs towards the centre of the rounded part, and exhibits 

 a well-developed primitive groove. In the anterior part the hypoblast 

 is not yet definite, the epiblast many-layered in the middle, the 

 mesoblast has not yet reached this region. The head-process is larger; 

 the primitive streak has the usual axial plate continuous with the 

 surface epiblast ; its lateral wings extend outwards between the epiblast 

 and here continuous hypoblast of flattened cells ; the mesoblast extends 

 outwards far beyond the termination of the hypoblast in the germinal 

 wall. In the hinder part of the streak, below the groove, there seems 

 to be an imperfectly united suture in the axial plate. The hypoblast 

 below this is continuous, but in the centre below the suture the ordinary 

 cells are replaced by a large coarsely granular cell. Here there is 

 indication of the lips of the anterior part of the blastopore. Below the 

 blastoderm are large, round, granular, formative cells of Balfour, 

 probably nutritive in function. 



In a specimen incubated for 66 hours, the posterior prolongation 

 was broader and less marked, the head-process more definite, a semi- 

 circular groove marked the position of the anterior boundary of the 

 future medullary plate, the suture in the streak has disappeared. 



From these stages it was evident that the primitive streak cannot 

 grow forwards from the posterior border of the area pellucida, as 

 generally described ; but is formed from before backwards, simultaneously 

 with an extension backwards in the form of a narrow bay of the area 

 pellucida. 



The head-process is merely the continuation forwards for a short 

 distance of the axial thickening of the lower layer which accompanies 

 the formation cf the primitive streak. 



The history of the mesoblast in the emu is summarized ; its founda- 

 tion is laid by the cells of the lower layer, and no part up to the stage 

 reached above is formed directly from epiblast. 



Subsequent stages arc more briefly described ; the appearance of 



