SEGMENTATION OP THE OVUM OF THE SHEEP. 217 



and trophoblast are shown to be derived from the same group 

 of segments, which are at an early period differentiated from 

 those which give rise to the epiblast. The trophoblast may 

 therefore, be said to be hypoblastic, and the hypoblast may be 

 said to completely surround the epiblast at one period of 

 development. 



Fig. 18 is a drawing of an embryo found in the uterus'of a 

 sheep of nine days. This specimen is very much larger than 

 any hitherto described. The vesicle was only slightly crumpled. 

 It was oval in shape, and the embryonic mass was plainly 

 visible on the longer face. 



Fig. 19 is a drawing of a section taken along the line a — b. 

 The trophoblast is quite plainly distinct from the embryonic 

 epiblast, which has grown very considerably, and forms a solid 

 lenticular mass. The hypoblast is stretched tightly against 

 the inner surface of the epiblast, and even extends some short 

 distance beyond the limits of the epiblast. It must be remem- 

 bered that fig. 19 is taken rather obliquely, and so gives the 

 impression of a further extension of hypoblast than really 

 exists. The hypoblast is of a remarkably close texture, which 

 is very unusual at such an early stage. 



The condition of the hypoblast as seen in fig. 19 may very 

 readily be derived from that seen in fig. 17 by the expansion 

 of the walls of the blastocyst round the embryonic pole. At 

 ten days there is no great change. I have only one specimen 

 of this age. It was considerably crumpled (fig. 20). The 

 embryonal area has a longer and shorter axis. 



Fig. 21 is a section of the embryonal area. The tropho- 

 blast (T) is very thin, and so much stretched that not more 

 than six nuclei overlie the epiblast (E). 



The only change to be noted in the epiblast is a greater 

 activity apparent, and the lower cells seem to show a tendency 

 to become arranged in a rather columnar or stellate fashion. 

 There is a layer of smaller cells on the inner surface of the 

 epiblast, and one of very much larger cells near the outer 

 surface, which may be compared to the state of things in the 

 corresponding stages of the pig or tupaia. The hypoblast is a 



