THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 129 



Fig. 28. — Photo-mlerograpli ( x about 76) of a blastocyst of the same 

 batcli as the preceding, '-io mm. in diameter. From a spirit specimen. 

 eg. Coagnhim. 



Fig. 29. — Photo-micrograph ( X about 75) of another l^lastocyst, 

 •45 mm. diameter, of the same batch as the preceding, biit taken 

 immediately after transference to the fixative. "Viewed from the upper 

 pole. y. b. Yolk -body seen through the unilaminar wall. 



Fig. 30. — Photo-micrograph ( x about 75) of a blastocyst of the same 

 batch as the preceding, about '39 mm. in diametei', in which the cellular 

 wall has not yet been completed over the lower polar region. 



Fig. 31. — Photo-micrograph (x 140) of a section of a blastocyst. 

 •39 mm. diameter, of the same batch as the preceding and at precisely 

 the same developmental stage, the cellular wall having yet to be com- 

 pleted over the lower polar region {l.x>.). In the blastocyst cavity is 

 seen the yolk-body [y. h.) partially surrounded by a mass of coagulum 

 [eg.). (D. viv., 2 B, 16 . vii . '01, m. = '39, ^. Picro-nitro-osmic and 

 iron-hasmatoxylin.) 



Fig. 32. — Photo-micrograph ( x 140) of another blastocyst, '41 mm. 

 in diameter, of the same batch as the preceding, also with the cellular 

 wall still alisent over the lower polar region. Shell-membrane "0075 mm. 

 in thickness, y. b. Yolk-body. c. g. Coagvilum. The cellvilar wall 

 comprises about 130 cells. 



Fig. 33. — Photo-micrograph (X 140) of a blastocyst of the same batch 

 as the preceding, with a complete unilaminar cellular wall. y. b. Yolk- 

 body, in contact with inner surface of wall, in the region of the upper 

 pole. 



Fig. 34. — Photo-micrograph (x 100) of a section of a blastocyst 

 •57 mm. in diameter, i. c. Internal cell. (D. viv., 29 . vi . '04, y. Picro- 

 nitro-osmic.) 



Fig. 35. — Photo-micrograph (x 100) of a section of a Itlastocyst, ^73 

 mm. diameter, of the same batch as the preceding, shell, ^0045 mm. 

 thick. 



Fig. 36. — Photo-micrograph (x 100) of a section of a blastocyst '66 

 mm. diameter, of the same batch as the pi'eceding. Lower hemisphere 

 opposite yolk-body {y. b.) formed of larger cells than upper. Hermann 

 fixation. 



Fig. 37. — Photo-micrograph (x 140) of section of an abnormal 

 vesicle, "397 mm. diameter of the same batch as the normal vesicles 

 represented in figs. 27-33. abn. large binucleate cell, regarded as a. 

 blastomere of the lower hemisphere which has failed to divide in normal 

 fashion, cf . text, p. 42. 



VOL. 5l), PART 1. NEW SERIES. 9 



