72 
ON A PLATYPUS EMBRYO, 
more or less distinct both from the somite and the lateral meso- 
derm. The lateral mesoderm is not split, the ventral ccelom 
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 16th and 17th 
somites owing to their oblique direction. The somites in this 
region no longer possess 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 the form of a 
strand of cells (to. t. ) slightly projecting from the intermediate 
cell mass. The ccelom (cm.) is of great lateral extent, and 
numerous endothelial vessels (h. v.) are visible in the outer part 
of the section. ( x 70. ) 
Fig. 11. — Transverse section through the blastopore (bl.) which is situated 
to one side of a longitudinal eminence at the anterior end of the 
primitive groove (cf. also fig. 23). The mesoderm is unsplit for 
a considerable 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 highly 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 tiie 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 {n'-n iv -). 
(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.) 
