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 l^lastopore (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 l^y 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 Ijeen observed by Kolliker* in 

 the rabbit, by Bonnetf in the sheep, by Zumstein:|: 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 witli 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 tlie 

 head process is connected with the ectoderm by a narrow median 

 strand exhibiting a similar appearance to that shown by Graf 

 Speell 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), 



* Sitzuiigsber. Phys-med. Gesellscliaft in Wiirzburg. 1883. 



+ His's Archiv. 1884. 



t Mesoderm. 8vo, Bern, 1887. 



§ Anat. Anz. iii. Jalirg. 1888. 



II Svo. Marburg, 1888. 



1i Anat. Anz. 1888, p. 319 (et fig. 2). 



