THE EAKLY DEVELOPMENT OF THE MARSUPiALIA. 21 



secondarily homolecithal and secondarily lioloblastic. The 

 Marsupial ovum presents itself to my mind as the victim of 

 tendencies conditioned by its ancestry, and in particular it 

 appears as if its inherited tendency to elaborate yolk had not 

 yet been brought into accurate correlation with the other 

 changes (reduction in size, intra-uterine development), which 

 it has undergone in the course of phylogen}". As the conse- 

 quence it manufactures more yolk than it can utilise, and so 

 linds itself under the necessity of getting rid of the surplus. 

 Whether or not a comparable elimination of deutoplasmic 

 material occurs in the ova of other Marsupials, future investiga- 

 tion must decide. I should be quite prepared to find variation 

 in this regard, correlated perhaps with the size of the egg*. 

 In the case of Phascolarctus, Caldwell gives the diameter of 

 the ovum as "17 mm., and his figure of a (horizontal ?) section 

 of the uterine ovum (here produced as text-fig. 1, p. 27) shows 

 a differentiation of the cytoplasmic body of that into vacuo- 

 lated and granular zones quite comparable with that of the 

 Dasyure ovum. From the few measurements of ova of other 

 marsupials that I have been able to make, it would appear 

 that the ovum of Trichosurus approximates in size to that of 

 Dasyurus, Avhilst that of Perameles and probably also that 

 of Macropus are smaller. Fx-om Selenka's figure I have 

 calculated that the ovum of Didelphys measures about 'ISx 

 '12 mm. in diameter. In the smaller ova it is quite likely 

 that yolk-formation may not proceed so far as in the relatively 

 large ovum of Das^'urtts. 



2. Maturation and Ovulation. 



The details of the maturation process have not been fully 

 worked out, owing to lack of material. As in the Eutheria 

 (Sobotta, Yan der Stricht, Lams and Doorme, and others), 

 the first polar body is separated off in the ovary, the second 

 apparently in the upper part of the Fallopian tube where 

 entrance of the sperm takes place. The first polar figure 

 (late anaphase observed, fig. 5) lies in the formative cyto- 



