314 



PROFESSOR J. COSSAR EWART. 



D. Early Developmental Stages in the Horse and Sheep.* 



If horses are descended from ancestors allied to Hyracotherium of the London 

 Clay, ample time has elapsed to admit of prenatal as well as postnatal variation in 

 directions different from those followed by even-toed Ungulates. It is hence probable 

 that almost from the outset horse embryos will differ from sheep embryos at a like 

 stage in the life-history. 



The Ovum, — In the Eutheria the ova, though always small, vary considerably ; 

 they may be under '1 mm. or over '3 mm. in diameter. For example, in deer the 



Text-fig. 8. — Segmenting ovum of sheep with distinct zona pellucida (s.p.). 

 Text-fig. 9. — Blastodermic vesicle of sheep, tr., trophoblast ; i.c.m., inner cell mass. Note 

 absence of zona pellucida. (After Assheton.) 



Text-fig. 10. — Ovum of deer. The inner cell-mass (text-fig. 9) has differentiated to form 

 (1) the embryonic shield {c.s. ) continuous with trophoblast (tr.), and (2) the endoderm 

 (end.). At the corresponding stage in the horse there is still a zona pellucida — the line ex- 

 ternal to the shield and trophoblast represents the zona in the horse. (After Keibel.) 



ovum may only measure '07 mm. ; on the other hand, the human ovum varies from 

 '22 to "32 mm. In sheep the ovum, exclusive of the zona pellucida, measures '15 mm. ; 

 in the horse it probably never exceeds '18 mm. In the ripe ovum of the sheep, 

 figured by Assheton, there is no indication of a perivitelline space ; but even in the 

 unripe ovum shown in fig. 2 from a 6-mm. ovarian follicle of a mare there is a 

 distinct space between the vitellus and the zona pellucida. 



In the sheep the zona (text-fig. 8, z.p.), attenuated at the end of the first week, 

 ruptures and disappears about the ninth day of gestation. As the amnion is only 

 completed at the end of the second or beginning of the third week, it follows that 



* For information about the development of the sheep I am mainly indebted to papers by Assheton and 

 Bonnet, more especially to "The Morphology of the Ungulate Placenta," Assheton, Phil. Trans. Roy. Soc, 1906; 

 "The Segmentation of the Ovum of the Sheep," Assheton, Quart. Jour. Med. Sci., 1898 ; " Beitriige zur Embryologie 

 der Wiederkiiuer," R. Bonnet, Archivf. Anat. u. Physiol., Anat. Abth., 1889. 



