SEGMENTATION OF THE OVUM OF THE SHEEP. 261 



PLATE 17. 



Fig. 23 (Ovis 40, No. 1). — Age 6 days (?). Segmenting ovum with fifteen 

 segments, whicli surrounded a central cavity comparable to tliat shown in 

 fig. 6 on Plate 15. 



Fig. 24 (Ovis 40, No. 2). — Age 6 days (?). An eight-segmeut embryo 

 separated into two groups of four cells. One segment contained certain 

 round bodies which are not found in any of the others. Stain borax carmine. 



Fig. 25 (Ovis 83, No. 1). — Age 2J days. Six-segment stage of sheep. 

 A section of this is shown in Qg. 4, Plate 15. 



Fig. 26. — Section of the side of an embryonal area of the pig of thirteen 

 days; to show the difference in the character of the epiblast and trophoblast, 

 at the place where the first sign of the amnion fold can be detected. 



Fig. 27. — Another section of a similar embryo of the pig. 



Fig. 28. — The whole section, from a part of which Fig. 29 was drawn. Pig 

 of about 17 days. The amnion and false amnion folds are seen. 



Fig. 29. — A section of a portion of the amnion fold of a pig of about 17 

 days. A very sudden alteration in the character of the epiblast of the amnion 

 between the letters T and E is supposed to indicate a transition from tropho- 

 blast to true epiblast of ihe embryonal area. 



PLATE 18. 



In the diagrams on this plate, pink indicates epiblast, green hypoblast. The 

 black ring is the zona radiata. 



Fig. a represents the ovum of a bird while iu the oviduct, showing the 

 segmentation cavity. Compare Duval (23), fig. 29. 



Fig. B. — The ovum of a bird about the time of laying. Compare Duval's 

 •Atlas,' fig. 33. 



Fig. C. — The ovum of a bird after a .few hours of incubation. The sub- 

 germmal cavity is now well established. Compare Duval's 'Atlas,' figs. 

 49—55, and Duval (23), 50—53. 



Fig. D. — A diagram to illustrate a supposed intermediate condition of a 

 protoentherian ovum in which a tendency to grow over the epiblast is 

 exhibited by cell masses of hypoblastic origin. 



Fig. W. — The segmenting ovum of a sheep and bat, and by hypothesis of 

 all placental mammals. 



Fig. X. — Another later stage of a segmenting ovum of Ovis, Tupaia, and 

 by hypothesis of some other mammals. The hypoblast has completely 

 surrounded the epiblast. 



