466 THE PHYSIOLOGY OF REPRODUCTION 



drain into lymphatic vessels in the outer half of the mucosa, 

 where also the arterioles and venules lie. All the blood-vessels 

 in the inner half are capillaries. In all probability the fertilised 

 ovum, during its sojourn in the Fallopian tube and while it lies 

 free in the uterine cavity, does not influence the structure of 

 the mucosa, and may implant itself at any period during the 

 cestrous cycle (Bryce and Teacher l ). But under the abnormal 

 conditions in a tubal pregnancy, the uterine mucosa undergoes 

 a decidual change although no fertilised ovum is embedded in it. 



In all the early specimens the ovum was completely en- 

 closed in the uterine mucosa, and the actual process of em- 

 bedding has not yet been observed. John Hunter considered 

 that the ovum reached the uterus from the Fallopian tube under 

 the mucous membrane, and so had a decidua reflexa, while at a 

 later stage the mucosa developed underneath it ; hence the 

 term decidua serotina. Sharpey supposed that the enclosure 

 was effected by circumvallation, i.e. by a growth round the 

 ovum of two folds of mucosal tissue, which fused and formed 

 the decidua capsularis. But v. Spee 2 discovered a different 

 mode of embedding in the guinea-pig, and later stated that it 

 was the same in Man, viz. a destruction of the superficial 

 epithelium, and the implantation of the ovum in the cellular 

 substance of the mucous membrane. This view has received 

 considerable support from the researches of v. Heukelom, 3 

 Peters, 4 Bryce and Teacher, and others. At the same time 

 it must be borne in mind that His, 5 in describing an early 

 human ovum in 1897, stated that the implantation cavity was 

 lined with epithelium, and thus represented a part of the 

 uterine lumen shut off by the growth of decidual folds. 



At the time of embedding, segmentation has probably 



1 Bryce and Teacher, The Early Imbedding and Development of the 

 Human Ovum, Glasgow, 1908. 



2 V. Spee, "Neue Beobachtungen iiber sehr f riihe Entwicklungsstufen des 

 menschlichen Eies," Arch. f. Anat. u. Phys., anat. Abth., 1896. 



3 V. Heukelom, " Ueber die menschliche Placentation," Arch. f. Anat. 

 u. Phys., anat. Abth., 1898. 



4 Peters, Ueber die Einbettung des menschlichen Eies, Leipzig u. Wien, 

 1899. 



5 His, "Die Umschliessung des menschlichen Frucht wahrend der 

 friihesten Zeit der Schwangerschaft," Arch. f. Anat. u. Phys., anat. Abth., 

 1897. 



