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



of optic lobes, and its ventral wall is still incompletely united 

 along the middle line. The intermediate cell mass is very clearly 

 distinguishable, but there is as yet no indication of the Wolffian 

 duct. The notochord is continuous behind with the floor of the 

 medullary canal ; at this point it is continuous with the hypoblast 

 at the sides. 



In an egg which had been incubated for a hundred and eighteen 

 hours, the blastoderm was found to be about seven and a half centi- 

 metres in diameter; the vascular area, still without developed blood- 

 vessels, was eight millimetres in length, thus being smaller than the 

 last. There were nineteen protovertebr^e. The heart and splanchno- 

 pleure folds were not further advanced in development ; the optic 

 lobes had just begun to bud out, and the amnion invested the 

 whole head end of the embryo. The neurenteric canal is not yet 

 distinguishable. From the hinder end of the medullary canal 

 (plate X. fig 7) there leads backwards a narrow passage which 

 opens on the surface by a rounded opening close to the posterior 

 termination of the area pellucida. In front of the opening are 

 two unsymmetrical rounded lobes. Just behind the posterior end 

 of the notochord there is a complete continuity betv/een the three 

 germinal layers. 



In an embryo of a hundred and fifteen hours (plate XL fig. 8) the 

 blastoderm was found to have spread over about a third of the 

 yolk ; the vascular area, which was twelve millimetres hi length, 

 was marked with numerous 'blood-islands' ; the area pellucida 

 was 7 mm. ; there were twenty-four pairs of protovertebrae, and 

 the cranial flexure was beginning to be distinct ; the optic vesicles 

 were prominent, and the head of the embryo was beginning to be 

 turned, so as to lie on the left side. 



The posterior end of the neural axis exhibited very nearly the same 

 general appearance as in the preceding stage, but the pear-shaped 

 vesicle terminated more abruptly behind. Some little distance in 



