IV.] "tHE AMNION. 83 



blastic in origin, having been separated from the epiblast at the epoch of the 

 apparent fusion of the epiblast and mesoblast in the region of the primitive 

 streaii or axia-cord of His. This view, prompted as it evidently is by theoretical 

 considerations, must be regarded as untenable, since the primitive streak has 

 nothing whatever to do with the permanent body of the embryo. 



Quite recently Romiti {Archiv. Microscopi Anat. x. 1874) has described the 

 Wolffian duct as being formed by an involution of the epitlielium of the pleuro- 

 peritoneal cavity, in the form of a longitudinal groove which is thrust up into 

 the superior portions of the intertnediate cell-mass, and the communication of 

 which with the pleuroperitoneal cavity is speedily obliterated. Such a mode 

 of origin recommends itself to the embryologist, inasmuch as it is certainly the 

 Way in which< as we shall see, the Wolffian duct is formed in Amphibia and 

 Osseous Fishes. For that very reason however it should be received with 

 caution ; all the more since the sections drawn by Komiti, and described as 

 supporting his views, evidently belong to a stage considerably later than that 

 at which the duct first distinctly appears. We hope to be able to shew, in the 

 second part of this work, that the mode of development of the Wolflaan duct 

 described above, and which we believe to be the real one, is not so abnormal as 

 it might at first sight be supposed to be. 



22. The amnion, especially the anterior or head-fold, 

 advances in growth very rapidly during the second day, and 

 at its close completely covers the head and neck of the 

 embryo ; so much so that it is necessary to tear or remove it 

 when the head has to be examined in hardened opaque speci- 

 mens. The tail and lateral folds of the amnion, though still 

 progressing, lag considerably behind the head-fold. 



23. The chief events then which occur during the 

 second half of the second day are as follows : — 



1. The second and third cerebral vesicles make their 

 appearance behind the first. 



2. The optic vesicles spring as hollow buds from the 

 lateral, and the vesicles of the cerebral hemispheres from the 

 front portions of the first cerebral vesicle. 



3. The first rudiment of the ear is formed as an involu- 

 tion of the epiblast on the side of the hind-brain or third 

 cerebral vesicle. 



4. The first indications of the cranial flexure become 

 visible. 



5. The head-fold, and especially the splanchnopleure 

 moiety, advances rapidly backwards ; the head of the embryo 

 is in consequence more definitely formed. The tail-fold also 

 becomes distinct. 



6. The curvature of the heart increases ; the first rudi- 

 ments of the auricles appear. 



7. The circulation of the yolk-sac is completed. 



8. The amnion grows rapidly. 



6-2 



