1882.] Blastodermic Layers in the Rat and Mouse. 43& 



of the blastodermic vesicle ; below this a layer of epiblast cells, while 

 still more internal is the hypoblast layer. These only extend over a 

 limited area of the wall of the vesicle, and it is still a disputed question 

 whether the epiblast forms the wall of the vesicle, or whether this is 

 formed by the covering layer. The vesicle above described for the rat 

 has all the elements described for the rabbit with this difference, and 

 herein lies the whole explanation of this form of development — that 

 there is an enormous development of the epiblast cells, which are 

 thrust, as it were, into the cavity of the vesicle and carry the hypo- 

 blast as a single layer of cells over their surface. This early stage, in 

 which the epiblast is solid and is not in the form of a cellular layer, is 

 succeeded in the course of a single day by a stage in which the solid 

 epiblast of the preceding stage is converted into an epiblastic vesicle 

 by the formation of a cavity in the interior of the solid mass. (Fig. 2.) 

 This epiblastic vesicle is not open either at the placental or free pole of 

 the ovum, and it has to be now noted that the embryonic area is not 

 formed at the placental pole of the ovum, where the epiblast cells 

 began first to multiply and protrude into the cavity of the blasto- 

 dermic vesicle, but adjacent to the free pole of the ovum, which the 

 epiblast, owing to its great development, has now almost reached 

 (covered of course by the hypoblast layer) . 



At this stage, in addition to the changes in the solid mass of epi- 

 blast, leading to the formation of a cavity in its centre, there has also 

 been a development of cells from the placental pole of the blastodermic 

 vesicle, which go on increasing during the course of development, and 

 which ultimately form a layer of large cells lying next the maternal 

 decidua. (Fig. 2, c.v.) In the course of another day the last-mentioned 

 stage is succeeded by one in which the single epiblastic vesicle becomes 



Fig. 2. 



Section of OTum from rat, 7 days 16 hours. 



