1887.] Embryology of Monotremata and Marsupialia. 



179 



ovary, in Marsupialia; but in an ovum of Phascolarctos, from the uterus, 

 I found a chorion like that of Monotremata, and surrounded moreover 

 by a thin transparent membrane — a shell- membrane. Within the uterus 

 the chorion, increasing in thickness, becomes transformed into an 

 albumen layer, and is eventually absorbed, passing through the 

 vitelline membrane to nourish the ovum, so that eventually the 

 vitelline membrane comes to be close to the shell. 



As in Monotremata, a coagulable, and, when coagulated, deeply 

 staining fluid makes its appearance between the vitelline membrane 

 and ovum (blastoderm). 



The shell-membrane persists until the developing ovum becomes 

 fixed to the walls of the uterus, after which it disappears. 



The paper then compares the egg-membranes just described with 

 those of Placentalia, and those of Vertebrata generally. 



2. Segmentation. 



The telolecithal ova of Monotremata and Marsupialia go through a 

 partial segmentation. The ova of Placentalia segment completely, 

 but the resulting blastodermic vesicle is identical with that produced 

 by partial segmentation in Monotremata and Marsupialia. 



In Monotremata there is a posterior lip to the blastopore similar to 

 that of Elasmobranchii. The epiblast grows in so rapidly from the 

 sides, that a primitive streak region is formed in front of the posterior 

 lip long before the epiblast has enclosed the yolk. This unenclosed 

 area in front of the primitive streak probably includes a region 

 where the hypoblast (yolk) has secondarily broken through the 

 epiblast. The existence of such a region wonld hide the position of 

 the anterior lip of the blastopore. The circumference of the circle 

 made up by the larger arc of the edge of the blastoderm on the yolk, 

 and the smaller arc of the posterior lip of the blastopore, is a 

 measure of the quantity of yolk in a merobla-stic ovum. 



In Marsupialia the epiblastic growth encloses the hypoblast at 

 a very early age, except over a narrow slit in front of the posterior 

 lip of the blastopore. This slit corresponds to the area enclosed by 

 the circle described above in a meroblastic egg. The primitive streak 

 is not conspicuous at an early age because of the large size of the 

 cells. No hypoblast projects through the epiblast in front of the 

 primitive streak region. I would explain the segmentation and the 

 gastrula of Placentalia in the same way. Balfour's objection (' Comp. 

 Embryol,' vol. 2, p. 187) to Van Beneden's original comparison of the 

 blastopore of the rabbit with that of a frog, is explained away by 

 the presence of a posterior lip to the blastopore in Marsupialia. My 

 explanation postulates the existence of a similar structure in the 

 rabbit. The blastopore of the rabbit corresponds therefore to the 

 whole area marked out by the growing epiblast and the posterior lip 



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