THE EARLY DEVELOPMENT OF THE MARSUPIAL! A. 133 



Fig. 72. — Drawing (x 630) corresponding to the above, from the 

 formative region of a 6 . 7 . '04 blastocyst, also showing two primitive 

 entodermal cells, evidently sister-cells. 



PLATE 8. 



Figs. 73, 74, 76. — Sections of the formative region of 6 . 7 . "04 blasto- 

 cysts, showing the attenuated shell-membrane, the unilaminar wall, and 

 in close contact with the inner surface of the latter, the piimitive ento- 

 dermal cells (ent.) (x 630). 



Fig. 75. — Section corresponding to the above, shoAving an entodermal 

 mother-cell (ent.), part of whose cell-body underlies the adjacent ecto- 

 dermal cell of the wall. The spheroidal inwardly projecting cell on the 

 left is probably also an entodermal mother-cell (x 630). 



Fig. 77. — Section ( x 630) of the non-formative region of a 6 . 7 . "04 

 blastocyst. 



Fig. 78. — Section ( x 630) of the embryonal area, and the adjoining 

 portion of the still unilaminar extra-embryonal region of a blastocyst of 

 the 5 . '01 stage, emb. ect. Embryonal ectoderm, ent. Entoderm, tr. 

 ect. Extra-embryonal ectoderm (tropho-ectoderm). The position of the 

 junctional line is readily recognisable. (D . viv. , 5, IS . vii . "01. Picro- 

 nitro-osmic and DelafiekVs hsematoxylin.) 



Fig. 79. — Section (x 630) through the corresponding regions in an 

 8 . vi . '01 blastocyst. Note the thickening of the embryonal ectoderm 

 (emb. ect.), and the peripheral extension of the entoderm (ent.) below 

 the tropho-ectoderm. (D. viv., 8 . vi . "01. Picro-nitro-osmic and 

 Delafield.J 



Fig. 80. — Section (x 600) through the formative (embryonal) region 

 of a blastocyst of P. nasuta, 13 mm. in diameter. It is thicker than 

 that of the Dasyure Ijlastocyst at the corresponding stage of develop- 

 ment ; the primitive entodermal cells are well marked. 



Fig. 81. — Section (x 600) corresponding to the above from another 

 1"3 mm. blastocyst of P. nasuta, of the same batch as the preceding, 

 but apparently very slightly earlier, the entodermal cells being stiU in 

 process of separating from the unilaminar wall. ent. Entoderm, tr. ect. 

 Tropho-ectoderm. 



PLATE 9. 



Fig. 82.— Section ( x about 430) of a section of a blastocyst of M. 

 ruficollis -35 mm. in diameter, showing the major portion of the 

 formative region (/. a.) and a small portion of the non-formative (//•. ect.). 



