288 PROFESSOR J. COSSAR EWART. 



examined the limbs of embryos hoping to find vestiges of the phalanges of the second 

 and fourth digits. Unfortunately, he failed to discover in the specimens at his 

 disposal evidence that in the modern horse the second and fourth digits are nearly 

 as complete during the earlier weeks of development as they were in the three-toed 

 Miocene horses. Since 1876, when Huxley lectured in New York on the evolution 

 of the Equidse, our knowledge of fossil horses has advanced so rapidly that we are 

 now familiar with almost every link in the chain connecting the one-toed Pliocene 

 horses with their remote four-toed Eocene ancestors. Further, we know that in 

 modern breeds the second and fourth digits of the fore-limbs bear during develop- 

 ment nearly the same relation to the third digit as in three-toed Miocene horses.* 



But while some progress has been made during recent years in working out the 

 development of the Equidse, we still know surprisingly little of the phases through 

 which" the horse passes during the earlier weeks of gestation ; as Assheton pointed 

 out some years ago, " a thorough investigation into the development of Equus and 

 its placenta has still to be made." t 



It is doubtless true that Hausmann published in 1840 an account of the earlier 

 stages of equine development,! and that in 1890 Prof. Martin of Zurich described 

 a 21-days horse embryo ; § but Hausmann's views are no longer accepted, and there 

 are good reasons for believing that Martin overestimated the age of his embryo. 



That Hausmann's contributions were of no great permanent value may be inferred 

 from the statements made by Bonnet in 1889, at the Berlin Conference of German 

 Anatomists. When discussing at this conference the foetal membranes of the 

 Equidae, Bonnet pointed out that Hausmann's account of the development during 

 the earlier weeks had caused more confusion than enlightenment, and that his 

 drawings of horse embryos are almost incomprehensible and worthless. 



Though in 1889 Bonnet was in a position to discuss at some length the fetal 

 appendages of a 28-days horse embryo, he was unfortunately unable to add 

 appreciably to our knowledge of yet earlier stages ; moreover, by asserting that the 

 rate of development in the horse varied greatly during the earlier weeks, Bonnet, 

 like Hausmann, has caused confusion rather than enlightenment. || 



In the Berlin paper Bonnet relates that he received from a pupil a blastocyst 

 taken from a mare 21 days after the first service, but that, owing to the rare specimen 

 having been subjected to a preliminary examination before it reached his hands, he could 

 only certainly make out that the blastocyst (fig. 3) had a globular form, measured 

 12 to 13 mm. in diameter, and was invested by a zona pellucida 4/a in thickness. 



A year after Bonnet's paper was read at Berlin, Prof. Paul Martin of Zurich 



* Ewart, " The Second and Fourth Digits in the Horse," Proc. Roy. Soc. Edinburgh, 1894 ; " The Limbs of the 

 Horse," Journ. Anat. and Physiol., Jan. and Feb. 1894. 



f "The Morphology of the Ungulate Placenta," Phil. Trans. Roy. Soc, vol. c, Ser. B, 1906. 



I Hausmann, Uber Zeugung und Enstehung das wahren weiblichen Eier bei den Sdugetieren, Hanover, 1840. 



ij Paul Martin, "Ein Pferdeei vom 21 Tage," Schweizer Archivfur Thierheilkunde, Zurich, 1890. 



|| Bonnet, "Die Erhaute des Pferdes," Verhandlungen der anat. Gesellschaft, Jena, 1889. 



