292 PROFESSOR J. COSSAR EWART. 



regarding Bonnet's 13-mm. blastocyst as representing the phase reached at the 

 beginning of the third week, and Martin's 35-mm. blastocyst as representing the 

 phase reached at the middle of the third week of development.* 



1. Bonnet's 13 -mm. Blastocyst (PI. IX, fig. 3). 



Sheep and goats are about as well developed and precocious at birth as foals. It 

 might hence be assumed that the rate of development in Ungulates bears, as a rule, 

 an intimate relation to the gestation period. In the Celtic pony the gestation period 

 is approximately 366 days,f in sheep it seems to be about 150 days, and in the pig 

 about 112 days; i.e. in the Celtic pony the gestation is practically three times that 

 of the pig and 2"2 times that of the sheep. But the examination of a collection of 

 Ungulate embryos clearly shows that the rate of progress, during at least the earlier 

 weeks, bears little, if any, relation to the length of the gestation period, and especially 

 that, though there is a family resemblance between the embryos of odd-toed and 

 even-toed Ungulates, the foetal appendages may differ profoundly. 



Further inquiries will probably show that the blastodermic vesicle of the sheep 

 at the end of the twelfth day agrees generally with the blastodermic vesicle of the 

 horse at the end of the fourteenth day, and that a 15-days sheep embryo % differs 

 but little from a 17- or 18-days horse embryo. But while at the outset horse embryos 

 differ but little from sheep embryos, there seem to be, almost from the first, differ- 

 ences in the foetal membranes. In Bonnet's 13-mm. horse blastocyst the zona pel- 

 lucida was smooth, resistant, and elastic, and had a thickness of 4/u..§ Though in 

 the sheep the blastocyst at the end of the twelfth day only measures about 1*5 mm., 

 the zona pellucida has completely disappeared — according to Assheton the zona is 

 greatly attenuated, if not actually absorbed, at or about the eighth day.|| Owing to 

 the 13-mm. blastocyst having been injured, Bonnet was unable to say anything about 

 the embryo it contained — he was only able to note the presence of isolated nucleated 

 polygonal cells adhering to the inner surface of the zona pellucida. In all probability 

 further inquiries will show that a horse embryo at the end of the second week is at 

 least as far advanced as a 1 0-day s sheep embryo. 



2. Martins 35-mm. Blastocyst (PI. IX, fig. 4). 



Martin's blastocyst not only differed from Bonnet's in shape and size, but 

 also in having a 4-mm. thick albumen coat (fig. 4, alb.) within which was found no 



* A 19-days horse embryo (text-fig. 19) in Hausmann's collection had ten mesodermic somites. Seeing 

 that Martin's so-called 21-days embryo had only four somites, it was probably under rather than over 18 days. 



t It is on record that the average gestation period for thirty-three thoroughbred mares of the Middle Park stud, 

 Eltham, was 335 - 5 days ; but in Shires and Clydesdales the gestation period seems to approach that of the wild horse 

 of Mongolia (357 days), while in the ass it may run to 385 days. 



{ Assheton has pointed out that in the sheep, goat, and pig " there is a close parallelism in time with reference 

 to the development of the embryo." (hiy's Hospital Reports, vol. lxii. 



§ That the zona pellucida of a 13-mm. equine blastocyst has a thickness of 4/j. wants confirmation. 



|| Assheton, "Segmentation of the Ovum of the Sheep," Quart. Journ. Micro. Sci., vol. xli, 1898, 



