224 RICHAED ASSHETON. 



type of development in which a central core of epiblast is sur- 

 rounded by a single layer of hypoblast cells. This group con- 

 tains Ovis, Sus, Tupaia;,Ta]paj Sorex, Lepus, and probably Cavia. 

 These all have a strong zona radiata. 



Fig. Y shows the next stage of all these forms, in which the 

 hypoblast has become doubled over the ab-embryonic pole. At 

 this point one of the number, Cavia, loses its zona radiata, and 

 branches off on a line of development of its own (Cj.^.) 



Continuing down the page is a figure (Z) which may be 

 supposed to be a stage common still to the remaining members 

 of this group, which, however, after this point take three rather 

 distinct courses. On the right side of the plate are shown 

 Lepus and Sorex, L^_4 and Sj_3. 



These two are characterised by the looser arrangement of 

 the epiblast, which condition results in a gradual expansion 

 and formation of a flat epiblastic disc. Both forms retain the 

 zona radiata until after the growth of the epiblastic disc has 

 caused the rupture of the overlying hypoblast cells (Rauber 

 layer). This leads to the inclusion (either permanent or 

 temporary) of these fragments among the epiblastic disc 

 cells. 



On the left of the page, diverging also from Z, is another 

 group containing Sus and Tupaia (Suj_5 Tu^_2), and probably 

 Ovis. These are characterised by the formation of a more 

 solid epiblastic knob, which does not become drawn out, but 

 undergoes a process of doubling up previous to assuming its 

 final disc-like form. The zona radiata is thrown off, so that 

 when the rupture of the overlying hypoblast cells occurs there 

 is nothing to prevent the fragments from being lost. Accord- 

 ingly none become incorporated in the epiblastic disc. 



Intermediate between these is placed Talpa (Tj_4), which is 

 like Tupaia and Sus as regards its epiblastic characteristics, 

 but like Lepus and Sorex retains the zona radiata, with the 

 result that the fragments of overlying hypoblast become incor- 

 porated with the epiblastic disc (Heape, 30). 



On turning again to the first figure, D, there will be found 

 upon the right of the plate a series of five figures, Mj_5, which 



