294 PROFESSOR J. COSSAR EWART. 



and the narrow medullary groove with four somites at each side are indicated, while 

 a transverse section through the posterior end of the primitive groove (text-fig. l) 

 makes it clear that the mesoderm in this region had not yet split into somatic and 

 splanchnic layers, and especially that very little, if any, progress had been made in 

 providing an amnion. 



In a summary Martin directs special attention to the 35-mm. blastocyst being 

 egg-shaped, to the presence of an albumen layer, and to the rudimentary condition of 

 the amnion. 



3. Hausmann s 19-clays Embryos. 



Hausmann, who had a stud including fifty-two mares, secured three 19-days 

 embryos, viz. one 19 days 45 minutes after service, one 19 days and 7 hours, and 

 one 19 days and 20 hours after service. Hausmann' s figures of these embryos are 

 reproduced in text-figs. 19 to 21. Though it must be admitted that these figures 

 are, as Bonnet says, almost incomprehensible, they afford conclusive evidence in 

 support of the view that Martin's embryo, instead of representing the stage reached 

 at the end of the third week, represents the stage reached on the seventeenth or 

 eighteenth day of gestation. Hausmann's 19 days 45 minutes embryo is characterised 

 by ten or eleven mesodermic somites ; Martin's embryo had only four somites (fig. 7)- 

 As my 21 -days embryo had over twenty somites (fie - . 10), it may be safely assumed that 

 Martin's embryo represents the stage reached about the middle of the third week. 



B. The Reproductive Organs and Foetal Membranes at the End 



of the Third Week. 



1. The Ovaries and Corpus luteum, Oviduct and Uterus. 



(l) The Ovaries and Corpus luteum. — The ovaries of the mare from which the 

 21 -days embryo was obtained are represented in figs. 12 to 15. Up to the time ova 

 begin to mature the ovaries in the mare are small and smooth, and more or less 

 kidney-shaped ; but as maturity is reached they increase in size, and eventually 

 present a number of prominences each indicating the position of a growing ovarian 

 follicle.* 



A number of follicular prominences are seen in figs. 12 and 13, while sections 

 through follicles, from which the contents escaped, are represented in fig. 15. 

 Evidently, during the three months the Highland mare was under observation, a 

 large number of ova were maturing with a view, doubtless, to giving her a chance 



* Under normal conditions fillies reach maturity — begin to discharge ripe ova — about the end of the second or 

 beginning of the third year, but under unfavourable conditions ovulation may only begin at the end of the third 

 year. On the other hand, when fillies are well fed during their first winter, maturity may be reached at the end 

 of the first or the beginning of the second year. Evidence of early maturity we have in a member of the Auchlochan 

 herd of Shetland ponies. This pony, born on May 7, 1907, had a well-developed vigorous foal on May 23, 1909 : 

 assuming the gestation period was 336 days, this filly became pregnant at the age of 1 year and 45 days. Seeing 

 that a heifer may become pregnant when only 5 months old, it is not surprising that a filly sometimes reaches maturity 

 before she is a year old. 



