326 PROFESSOR J. COSSAR EWART. 



A notochord (fig. 33) and branchial arches (text-fig. 6) represent a skeleton, and 

 excretory organs are represented by Wolffian bodies and pronephric ducts. 



In a comparison between horse and sheep embryos it is pointed out that even at 

 the stage characterised by four pairs of mesodermic somites the horse differs from 

 a sheep at the corresponding phase in its life-history, and. that, as development 

 proceeds, the differences, more especially in the foetal appendages, become more 

 pronounced. 



EXPLANATION OF PLATES. 



Plate IX. 



Fig. 1. Ovarian follicle of a mare containing an unripe ovum; this follicle only measured 6 mm. 

 The large follicle in fig. 16, which contained a ripe ovum, measured 37 by 33 mm. 



Fig. 2. The unripe ovum seen in fig. 1. x 300. This ovum measured - 11 mm. exclusive of the 

 zona pellucida ; the average size of the ripe ovum of the mare, exclusive of the zona pellucida, is probably 

 •18mm. In the sheep the ovum is about T5 mm. In deer the ovum maybe under *1 mm.; in Dasyurus 

 it averages '24 mm. ; while the human ovum varies from - 22 to *32 mm. ; hence the ovum in the horse, as in 

 the sheep, is relatively large. Between the ovum and the zona pellucida (z.p.) is the perivitelline space (p.s.) ; 

 the cells external to the zona form the corona radiata (c.r.). The egg of amphioxus only measures T2 mm., 

 yet it contains sufficient deutoplasm to provide all the energy required for development up to the gastrula 

 stage. It may hence be assumed that the ovum of the mare contains all the nutrient required for develop- 

 ment up to at least the morula stage. 



Fig. 3. This figure indicates the actual size of Bonnet's so-called 21-days blastocyst — it probably 

 represents the phase reached at the end of the second week. Bonnet states that the blastocyst was invested 

 by a zona pellucida 4/x thick. 



Fig. 4. Martin's so-called 21-days blastocyst, natural size, invested by a thick (3 to 4 mm.) albumen 

 layer (alb.). This 35-mm. blastocyst probably represents the stage reached at the middle of the third 

 week. After Martin. 



Fig. 5. The blastocyst, natural size (50 mm.), taken from a Highland mare 21 days and 2 hours after 

 the one and only service on the seventh day of oestrus. Note the difference in the size and position of the 

 embryo from the embryo in fig. 4, the left vitelline artery, the sinus terminalis, and the vitelline veins. 



Fig. 6. A section through the lower end of the left uterine horn. The blastocyst is kept in contact 

 with the tongue-like processes of the uterine mucosa by hydrostatic pressure. Natural size. 



Fig. 7. A dorsal view of Martin's so-called 21-days embryo. Note the four pairs of mesodermic 

 somites, the primitive streak, and the primitive and neural grooves. x 15. After Martin. 



Fig. 8. The 21-days embryo seen in fig. 5. This hook-shaped embryo is three times the size of, and 

 much further advanced in its development than, Martin's so-called 21-days embryo. Note the right 

 vitelline vein (v.v.) on its way to the relatively large heart (ht), the fronto-nasal process (f.p.), the four 

 branchial arches (b.a.) and branchial grooves, and the depression between the first arch and the fronto- 

 nasal process, which represents the mouth or stomodseum (s.). Only the front part of the amnion (am.) is 

 represented. x 15. 



Fig. 9. Part of the same embryo with its amniotic investment. Note the swelling near the end of the 

 frontonasal process due to the optic vesicle (o.ve.) ; the mouth between the fronto-nasal process and the 

 first pair of arches (1 b.a.) ; and the cavities of the three branchial pouches (b.p.). x 15. 



Fig. 10. A dorsal view of the same embryo. Note the mesodermic somites and the neural canal with 

 its opening to the exterior at the caudal end (n.p.). x 15. 



Fig. I 1. The same embryo seen from below, the anterior part surrounded by the amnion (am.). Note 

 the openings of the otic vesicles (o.v.), the great width of the pericardial chamber, the somites, and the fin- 

 like caudal expansion which consists in part of allantoic mesoderm. x 15. 



