STUDIES ON THE DEVELOPMENT OF THE HORSE. 297 



the third week, there was — if one may judge by the presence of leucocytes and fat 

 globules — abundance of the uterine milk so essential during the earlier weeks for the 

 nourishment of the embryo. 



2. The Blastocyst. 



It has already been mentioned that the blastocyst is globular in form and measures 

 13 mm. at the beginning of the third week; that it is ovoid and has a length of 

 35 mm. at the middle of the third week ; and that by the end of the third week it is 

 pear-shaped and measures 50 mm. (fig. 5). 



Though the blastocyst is not, as Bonnet stated in 1889, spherical up to the 

 seventh week, a globular form is retained longer in the Equidae than in even-toed 

 Ungulates ;* in an embryo pig, at the same phase as a 21-days horse, the blastocyst, 

 instead of measuring 50 mm., may reach a length of 1000 mm. Bonnet believed 

 there was still a trace of the zona pellucida at the end of' the fourth week, but 

 Martin failed to find any evidence of the zona in his 35-mm. blastocyst, and there was 

 no vestige of a zona at the end of the third week. As soon as the 21-days blastocyst 

 was exposed the embryo was seen at the broad rounded end (fig. 5), and there were 

 faint indications of the vitelline vessels and the sinus terminalis. Immediately 

 beyond the sinus it was possible to detect minute circular projections (trophoblastic 

 discs), and still nearer the small end shallow semi-opaque depressions (fig. 34). 



3. The Trophoblast. 



The isolated nucleated polygonal cells seen by Bonnet adhering to the inner 

 surface of the zona pellucida of his 13-mm. blastocyst doubtless belonged to the 

 trophoblast. The trophoblast of Martin's 35-mm. blastocyst apparently consisted 

 throughout of epiblastic cells almost tall enough to rank as columnar cells ; as is 

 often the case with trophoblastic cells, the nucleus was nearer the inner than the outer 

 end. In my 50-mm. blastocyst, in the absence of the albumen coat present in the 

 35-mm. blastocyst, the trophoblast lay in direct contact with the lining of the uterus. 

 As in this embryo the amnion was complete and the unsplit mesoderm highly 

 vascularised, I expected to find the trophoblast at least as highly specialised as in 

 Marsupials at a corresponding stage of development. 



Sections through the blastocyst in the embryonic area, in the region of the sinus 

 terminalis, and in the non-vascular distal end made it evident that three kinds of 

 epiblastic cells took part in forming the trophoblast, viz. : (a) typical columnar 

 cells, (b) very tall columnar cells, and (c) columnar cells ending in free sac-like 

 processes. The greater part of the trophoblast consisted of cells of the first category, 

 i.e. of fairly tall typical polygonal columnar cells in contact with each other along 

 their entire length, with deeply placed nuclei and nearly square-cut outer ends 



* Figures of the blastocyst at the end of the fourth, fifth, sixth, and seventh weeks are given in the writer's 

 pamphlet, A Critical Period in the Development of the Horse, A. & C. Black, 1897. 



