356 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



eggs. One of these became enclosed in the albumen of an opossum 

 egg (a, fig. 29, pi. 7) where it was found 24 hours later when the 

 uterus was opened. 

 The general course of development may now be briefly outlined. 



THE EGG ENVELOPES. 



In the Fallopian tube the spermatozoon enters the egg and fertili- 

 zation takes place. The albumen is then laid down about the ovum, 

 being deposited in laj^ers (figs. 27, 28, and 33, pis. 7 and 8) and 

 attaining a considerable thickness (figs 35 and 37, pi. 8.). Finally 

 around this there is added the shell membrane which characterizes 

 the egg of marsupial mammals (sm, figs. 26 to 28, pi. 7). These egg 

 envelopes, of course, are homologous to albumen and shell membrane 

 of the bird's egg ; marsupials are the highest mammals in which they 

 are present as vestigial structures. The writer has advanced the 

 theory that the shell membrane serves a useful purpose in insulating 

 vesicles from each other, for in later stages they are very closely and 

 tightly packed together in the uterus. The shell membrane may thus 

 serve to prevent the fusion of the chorions and the possible production 

 of sterile individuals ("free-martins"). The shell membrane is at 

 any rate a most remarkable structure: It is devoid of Cells yet it 

 grows in thickness and a thousand or more times its original area. 



Sometimes two ova are inclosed by the same egg envelopes; this 

 results in a 2-yoked egg (figs. 30-32, pi. 7), as often seen in hen's 

 eggs. Since only eight such cases have been found among some 

 thousands of eggs, double-yoked eggs must be very rare. This raises 

 an interesting question as to the mechanism by which the eggs are 

 distributed in the Fallopian tube so as normally to prevent their 

 inclusion together in the same egg envelope. 



CLEAVAGE. 



Cleavage begins in the uterus. In this particular, marsupials 

 differ from other known mammals, for in these the egg undergoes 

 segmentation in the Fallopian tube attaining at least the 2-celled 

 stage, more often the morula stage, before reaching the uterus. 



The 4-celled opossum egg shows an arrangement of cells much 

 as in the higher mammals (fig. 26, pi. 7) . 



The further cleavage of the marsupial egg differs from that of 

 other mammals in the absence of the morula or mulberry stage; for 

 as the cells multiply they arrange themselves from the start in the 

 form of a hollow sphere (figs. 27 and 28, pi. 7). This may already 

 be seen in the 16-celled stage. When the 50-celled stage is reached 

 the ovum is a completed vesicle, a perfect sphere ; it is hollow, except 

 for the excess of yolk which characterizes the marsupial egg. The 



