598 ON THE EARLY STAGES IN THE DEVELOP.ME.VT OF THE EMU, 



Plate X. 



Fig. 5. — Embryo of 65 hours with eleven pairs of protovertebrse and 

 developing vascular area. The vesicles of the brain have become 

 differentiated, with slight rudiments of the optic vesicles of the 

 fore-bi"ain (f.h.), the hind-brain (li.h.) beginning to divide into 

 two parts. The cleft in the fore-brain has become artificially 

 enlarged. ht, heart, v. vitelline vein. 



Fig. 6. — Hinder part of embryo of 9-t hours, with seventeen pairs of pro- 

 tovertebrse. pr. remains of primitive streak. 



Fig. 7. — Hinder part of embryo of 118 hours with nineteen pairs of proto- 

 veitebrse, in which there is a narrow canal leading from the 

 posterior end of the medullary canal and opening by a small 

 pore (o.) on the surface. From fresh specimen. 



Plate XI. 



Fig. 8. — Embryo of 115 hours with twenty-four pairs of protovertebrse, 

 with well-developed optic vesicles and S -shaped heart. The 

 dark spot (n.) marks the position of the neurenteric canal, pr. 

 hinder end of remains of primitive streak. From fresh specimen. 



Plate XII. 



Fig. 9. — Transverse section of blastoderm with a rudimentary primitive 

 streak (incubated for 51 hours) in the anterior part of the primi- 

 tive streak, showing union of upper and lower layers in their 

 thickened middle part. ep. epiblast. hy. developing hypoblast 

 cells, ins lower layer cells which will become converted into 

 stellate mesoblast. 



Fig. 10. — Transverse section of 70 hours' blastoderm (fig. 1), passing throut^h 

 the head process, ep. epiblast. I. lower layer cells of head 

 process, some of which are being converted into stellate mesoblast 

 cells, and others (hy.) into flattened hypoblast cells, n. 

 nutrient corpuscles. 



Fig. 11.— Section of the same blastoderm at the anterior end of the primi- 

 tive streak, ep. epiblast. ms. mesoblast cells developed from 

 lower layer (compare with fig. 9). hy. hypoblast cells, n, 

 nutrient corpuscles. 



