72 ON A PLATYPUS EMBKYO, 



more ov less distinct both from the somite and the lateral meso- 

 derm. The lateral mesoderm is not split, the ventral ecelom 

 being absent in this region. The other relations are the same as 

 in fig. 8. (X 70.) 



Fig. 10. — Transverse section passing through both the 16fch and 17th 

 somites owing to their oblique direction. The somites in this 

 region no longer po'-scss distinct cavities — they consist of an 

 upper and lower layer connected with each other by processes of 

 the cells. Between the somites and the lateral mesoderm there 

 is present the Anlagen of the Wolffian tubules in tlie form of a 

 strand of cells {w. /.) slightly pi ojecting from the intermediate 

 cell mass. The eoalom (ftf.) is of great lateral extent, and 

 numerous endothelial vessels {h. r.) are visible in the outer part 

 of the section. ( x 70.) 



Fig. 11. — Transverse section through the blastopore [hi.) which is situated 

 to one side of a longitudinal eminence at the anterior end of the 

 primitive gi-oove {rf. also fig. 23). The mesoderm is unsplit for 

 a consideral)]e distance out. ( x 70 } 



Fig. 12. — Transverse section through the primitive groove {pr. g.) slightly 

 behind fig. 11. (x 70.) 



Fig. 13. — Transverse section through the hinder region of the primitive 

 streak [pr. s. ). ( x 70. ) 



Fig. 14. — Median portion of fig. 2 more liighly magnified. The section 

 passes through the anterior end of the notochord where it 

 spreads out and is represented by an axial thickening of the 

 entoderm. With this thickening the mesoblast is continuous 

 laterally, (x 280.) 



Fig. 15. — Longitudinal section of anterior end of embryo passing through 

 the optic groove (op. gr.) and the head fold. ( x 180.) 



Fig. 16. — Longitudinal section of anterior region of the embryo, passing to 

 one side of the median line, especially to show the mid- brain 

 neuromere (xV.) and the four hind-brain neuromeres (7i'-?/,^'^). 

 (x 70.) 



Plate XII. 



Fig, 17, — Transverse section through the anterior portion of the 4th 

 neuromere showing the mesial and lateral bulgings on its under 

 side— the lateral one much the larger of the two and projecting 

 outwards, (x 180.) 



Fig. 18. — Longitudinal section through the 3rd and parts of the 2nd and 4th 

 neuromeres especially to show their arc shaped form. ( x 280.) 



