62 
ON A PLATYPUS EMBRYO, 
enlargement already referred to which projects as an elongated 
eminence composed of rounded cells at the anterior end of the 
primitive groove (figs. 11 and 23). In the hollow at one side of 
this eminence the blastopore (bl.) is situated. The blastopore 
leads into the blastoporic canal which runs forwards in the head 
process for a distance of 16 mm., and opens by a lateral 
opening into the cavity of the blastodermic vesicle (fig. 21, 
bl. op.). The inner opening of the blastoporic canal appears 
as a break in the entoderm. The walls of the canal are 
wholly composed of mesoderm. The lumen of the blastoporic 
canal is not a single one, but is divided up by bridges of 
cells into two or three smaller canals (fig. 22, bl. c). A similar 
duplication of the canal has been observed by Kolliker* in 
the rabbit, by Bonnetf in the sheep, by ZumsteinJ in the chick, 
and by Spee§ in the guinea-pig; hence the latter observer regards 
it as probably of general occurrence in the formation of the 
chorda in mammals. 
The head process runs forwards from the front end of the 
primitive streak, distinct from the ectoderm though closely applied 
below it. Laterally it is continuous at intervals with the meso- 
derm, while below it exhibits traces of cellular connection with 
the entoderm. This connection with the entoderm is interesting 
in view of the observations of Carius|| that the head process is 
free at first and only subsequently unites with the entoderm. 
Just anteriorly to the inner opening of the blastoporic canal the 
head process is connected with the ectoderm by a narrow median 
strand exhibiting a similar appearance to that shown by Graf 
SpeeH for the guinea-pig. Two small lumina at this stage not 
continuous with the blastoporic canal occur in the head process 
just anterior to the inner opening of the canal (fig. 20, nch. c), 
* Sitzungsber. Phys-med. Gesellscbaft in Wilrzburg. 1883. 
+ His's Archiv. 1884. 
£ Mesoderm. 8vo, Bern, 1887. 
§ Anat. Anz. iii. Jahrg. 1888. 
|| 8vo. Marburg, 1888. 
IF Anat. Anz. 1888, p. 319 (et fig. 2). 
