ATTACHMENT OF MAMMALIAN EMBRYO TO UTERUS. 189 



factor in moulding the shape of" the vesicle. In the rabbit the 

 vesicle attains a far greater size than in the dog before 

 becoming attached to the uterus^ though this may, no doubt, 

 also be partly owing to the far greater resistance offered by the 

 uterus of the dog. 



(vi) When the lumen of the uterus is large, and where the 

 investing coat is present, but delicate, the blastodermic vesicle 

 may grow to a great length, and possibly the development of 

 villi and consequent placental attachment may be brought 

 about by local developments of regions of pressure. 



(vii) Even in the rabbit, after the rupture of the albumen 

 layer the blastodermic vesicle may become extended along the 

 cavity of the uterus to a length of over 20 mm. 



EXPLANATION OF PLATE 19, 



Illustrating Mr. Richard Assheton's paper '' On the Causes 

 which lead to the Attachment of the Mammalian Em- 

 bryo to the Walls of the Uterus." 



List of Refekence Letters. 



a. Epiblastic papilla. ALB. Albumen layer, b. Epiblastic papilla. C. 

 Cavity of uterus, c. Epiblastic papilla. C. BL. Cavity of the blastodermic 

 vesicle, e. Wall of the blastodermic vesicle. EC. Ectoplacental cells. EL. 

 External longitudinal muscle layer. EMB. Embryonic area. EP. Epiblast. 

 H. Horn of the blastodermic vesicle. HT, Hypoblast. IC. Internal circular 

 layer of muscle-fibres. M. Mesometrium. MES. Mesoblast. P. L. Placental 

 fold of the uterine mucous membrane. OP. L. Obplacentai fold of uterus. 

 PP. L. Periplacental fold of uterus. UT. Cavity of the uterus. UT. EP. 

 Epithelium of the uterus. Z. Non-cellular continuation of horn of vesicle 

 attached to the remains of albumen coat. 



EiG. 1. — Transverse section of the uterus of a rabbit before any distension 

 has been caused by the presence of an embryo. 



EiG. 2. — Transverse section of the uterus of a rabbit on the seventh day 

 after fertilisation. The distension is chiefly at the expense of the weaker 



