SEGMENTATION OF THK OVUM OF THE SHBE?. 259 



DESCRIPTION OF PLATES 15—18, 



Illustrating Mr. Richard Assheton's paper " On the Segmen- 

 tation of the Ovum of the Sheep, with Observations on the 

 Hypothesis of a Hypoblastic Origin for the Trophoblast." 



List of Hefeeence Letteks. 



E. Epiblast. H. Hypoblast. M. Mesoblast. M. H. L. Membrana liypo- 

 blastica limitans. T. Troplioblast. Y. Spherical bodies found near the 

 surface of the uusegmented egg. Z. Zona radiata. 



PLATE 15. 



Fig. 1 (Ovis 3, No. 1). — Unfertilised ovum from the uterus. 



Fig. 2 (Ovis 3G, No. 1).— Age 1^ days. Section of the fertilised ovum of 

 a sheep, showing male and female pronucleus. Found in tiie Fallopian tube. 

 Preserved in "5 per cent, chromic acid. Stain carmalum. Thickness of 

 section -0075 mm. x 420. 



Fig. 3 (Ovis 35, No. 2). — Age 3 days. Section of the fertilised ovum of a 

 sheep. Fertilisation completed, and the division of the ovum is taking place. 

 Preserved iu 1 per cent, chromic acid. Staiu, Klein, hsematox. Thickness 

 of section -0075 mm. x 380. 



Fig. 4 (Ovis 33, No. 1).— Age 2| days. Section of a segmented ovum of 

 a sheep with six segments. Preserved weak Flemming. Stain carmaluiii. 

 Thickness of section "0075 mm. x 380. 



Fig. 5 (Ovis 49, No. 1).— Age between 4 and 6 days. Section of a 

 segmented ovum of a sheep with eight segments. Stain carmalum aud pif^nc 

 acid. Thickness of section '006 mm. x 380. 



Fig. 6 (Ovis 63, No. 1).— Age 4 days. Section of a segmented ovum of a 

 sheep with sixteen segments. Stain carmalum. Thickness of section 

 •005 mm. X 380. 



Fig. 7 (Ovis 33, No. 2).— Age 2^ days. Section of an ovum of the sheep 

 with eight segments. Preserved in '5 per cent, chromic acid. Stain car- 

 malum. Thickness of section "0075 mm. x 420. 



Fig. 8 (Ovis 18, No. 1).— Age 4 days. Section of the ovum of a slieep 

 with thirty segments, six of which are large and light in colour. Preserved 

 in "5 per cent, chromic acid. Stain carmalum. Thickness of section "OOo mm- 

 X 420. 



Fig. 9 (Ovis 14, No. 1).— Age C days. Section of the ovum of a sheep in 

 which the large light -coloured cells have been completely surrounded by the 



