I CARL GOTTFRIED HARTMAN 



5. The 8- to 16-celled stages '. 30 



a. Third and fourth cleavages 30 



1) General statement 30 



2) Cleavage indeterminate 31 



3) The 16-celled egg a blastocj^st 33 



b. The eliminated yolk 37 



c. Absence of polar differentiation 38 



PART II. THE FORMATION OF THE BLASTOCYST 



6. The early unilaminar blastocyst 42 



a. The just completed blastocyst 42 



1) Formation of the blastocj'st 42 



2) First appearance of polarity 43 



3) Inclusions within the cavity 44 



b. Cells included within the blastocyst cavity 46 



7. The formation of the entoderm 49 



a. Entoderm formation as conjectured by Selenka 49 



b. Entoderm formation in Dasyurus 50 



c. Formation of entoderm in the opossum 52 



d. Comparison of early and later blastocysts and growth of egg 56 



8. General summary 58 



INTRODUCTION 



a. Statement of the problem 



The desirability of increasing our knowledge of early mar- 

 supial development has frequently been emphasized by students 

 of mammalian embryology. The almost yolk-free condition of 

 the Eutherian egg, with its consequent holoblastic type of cleav- 

 age, is believed to have developed secondarily from a telolecithal 

 and distinctly meroblastic ovum. This latter condition obtains 

 in the monotreme egg, which may be considered transitional 

 between the Sauropsida and the two higher classes of Mammalia. 

 The large yolk-mass and the shell of the marsupial egg as well 

 as other considerations make it highly probable that the Mar- 

 supials are intermediate between the Monotremes and the true 

 Placentalia. It is reasonable, therefore, to expect the study of 

 the marsupials to contribute to the solution of many problems 

 in mammalian embryology which have arisen from the already 

 very extensive study of Eutherian development. 



Observations on the early development of marsupials have 

 been very few. Up to 1910, when Hill pubUshed his excellent 



