STUDIES ON THE DEVELOPMENT OE THE HORSE. 293 



trace of the zona pellucida. Martin thought the albumen coat was perhaps partly 

 formed in the oviduct, and that it provided nourishment as well as protection. 

 Though Bonnet makes no reference to an albumen coat in his 13-mm. blastocyst, it 

 is conceivable that the material for this coat is in part, as Martin suggests, provided 

 by the oviduct ; but in all probability the albumen is mainly derived, as in the mole, 

 from the uterus.* On making sections through the albumen layer, Martin ascertained 

 that it consisted of numerous strongly refractive delicate lamellae, amongst which 

 were groups of cells, free nuclei, and spermatozoa at various stages of disintegration.! 

 There is no evidence that the albumen coat served to fix the blastocyst to the uterine 

 mucous membrane. 



Martin describes the blastodermic vesicle as consisting of two quite distinct 

 layers — an ectodermal layer composed of columnar cells which seen from the surface 

 appear polygonal, and an endodermal layer of flat polygonal cells between which are 

 many gaps of a considerable size. In the neighbourhood of the embryo the blastocyst 

 consisted of three layers, an outer, inner, and middle, the last made up of round cells 



ec 



end 



Text-fig. 1. — Transverse section of Martin's embryo, am., amnion fold ; ec, ectoderm ; end., endoderm ; ms, mesoderm ; 

 pr., primitive groove. Compare with section of 21-days embryo (text-fig. 12). 



continuous with the embryonic mesoderm. When the blastocyst was removed from 

 the uterus, the embryo, being transparent, could not be detected, but as the fixing 

 proceeded an oblong structure resembling the sole of a shoe made its appearance. 

 As fig. 7 indicates, this embryo, rounded in front and pointed behind, had four somites 

 and was separated by a narrow area pellucida from the area opaca. In a pig embryo 

 with four mesodermic somites the amnion is represented by head, tail, and lateral 

 folds, and there is a mass of mesoderm lying under the tail-fold which in course of 

 time takes part in forming the allantois. Though Martin's embryo was 3 "25 mm. in 

 length, the amnion was apparently only represented by short inconspicuous lateral 

 folds (text-fig. 1, am.), and there was no rudiment of an allantois. 



That Martin's embryo represents a very early phase in the development of the 

 horse — a phase reached in the pig on the fourteenth or fifteenth day — is made evident 

 by the figures which illustrate his paper. In a dorsal view of the embryo (fig. 7) the 

 primitive streak, the broad pointed primitive groove, the position of Hensen's node, 



* Heape, " The Development of the Mole," Quart. Journ. Micro. Sci., vol. xxiii, 1888. 



t In Marsupials, as Professor Hill states, the ovum during its passage down the oviduct " becomes surrounded 

 by a transparent layer of albumen '015 to - 022 mm. in thickness, composed of very delicate concentric lamellae, and 

 having normally numbers of sperms embedded in it," and that this albumen layer is invested by a double-contoured 

 membrane comparable to the shell membrane of the Monotreme egg. J. P. Hill, " The Early Development of the 

 Marsupialia," Quart. Journ. Micro. Sci., vol. lvi, December 1910. 



