THE EAKLY DEVELOrMENT OF THE MAh'Sri'IALTA. 131 



Fig. 49. — Photo-micrograph ( x 150) of a corresponding preparation 

 of a vesicle of the same batch as the preceding, in which the -vvaTy and 

 irregularly thickened free edge of the entoderm (ent.) practically 

 coincides -^ath the junctional line and so conceals it from view. 



Fig. 50. — Photo-micrograph (x 150) of an in toto preparation of a 

 vesicle (8 . vi . '01 batch) viewed fi'om the inner surface as in the corres- 

 ponding preceding figures. The entoderm in the region of the embryonal 

 area has been removed, so that one sees the inner surface of the embryonal 

 ectoderm (emh. ect.) ; it is still in situ, though not in a quite intact con- 

 dition over the adjoining portion of extra-embryonal ectoderm. The 

 entoderaa has not yet extended over the region indicated liy the reference 

 line to tr. ect., so that hei*e the extra-embryonal ectoderm is clearly 

 visible. The jrmctional line is apparent. (D. viv.. 8 . vi . "dl. Picro- 

 nitro-osmic. Ehrlich's ha;matoxylin.) 



Fig. 51 (Plate 3).— Photo-micrograph (x 310) of a section of a 30- 

 celled egg of Perameles obesula; egg h. '24 x '23 mm. diameter, 

 showing the unilaminar layer formed by the Ijlastomeres. 



Fig. 52 (Plate 3). — Photo-micrograph (x 240) of a section of a 

 l)lastoeyst of P. nasuta '29 x "26 mm. diameter, showing the shell- 

 membrane (.•5.//*.), zona iz.p.). and the unilaminar cellular wall. The 

 portion of the latter adjacent to the I'eference lines is composed of 

 smaller Ijut thicker cells than the remainder. 



PLATE 6. 



Figs. 53 and 54. — Drawings ( X 84) of a 6-celled egg '34 mm. diameter, 

 fig. 53 showing a side view and fig. 54 a view from the lower pole. 

 Observe the characteristic ring-shaped arrangement of the Ijlastomeres. 

 y. h. Tolk-body, the shell-membrane, albumen layer with spenns in- 

 cluded, and the zona are readily distinguishable. Outlines drawn with 

 the aid of the camera lucida immediately after transference of the egg 

 to the fixing fluid. (D . viv., 22, 16 . vii . '01.) 



Figs. 65 and 56.— Drawings ( x about 88) of a 16-celled egg (about '37 

 mm. diameter) as seen from the side and lower pole respectively, from 

 the same batch as the eggs represented in figs. 24, 25, and 26. The charac- 

 teristic arrangement of the blastomeres in two superimi^osed, open 

 lings (each of eight cells) and the difference in size between the cells of 

 the two rings are evident. The irregular body {e.g.) seen in the cleavage 

 cavity in fig. 56 is a mass of coagiilum. Drawn from a spirit specimen. 

 The albumen layer as represented in fig. 56 is too thick. (D. viv., 

 3 B, 26 . vi . '01.) 



Figs. 57 and 58. — Drawings (x aljout 85) of a 12-celled egg ("38 mm. 

 diameter) as seen from the side and lower pole respectively. Fovir of 



