22 



K. Mfl'SUKlTi,'! 



of blood-vessels. These two lohcs of flu- hii'u'i'i' |>:H't ni:i\- In- ciilUnl 

 respeetively tin* ri^-ht, and the left lobe, wliilc the smullci- linlf of 

 the allantoic vesicle may be called the niidillc lobe. 



The 1 wo constrictions that divide the middle lobe from tlic larger 

 half of the allantoic vesicle are caused in two different ways. The 

 anterior constriction is ^ery easy to explain. It was mentioned 

 above that one set of the allantoic vessels runs on the anterior side 

 of the as yet small allantc^ic vesicle. Now, in the rapid growth of the 

 vesicle, the lines along which blood-vessels run cannot, on account of 

 their presence, keep up in their growth with the rest of the vesicle, 

 aiid ai'c iicccssaiMl V left behind until along these lines f hei-e are pi'odn(_'ed 

 groo\es at tlie bottom of whicli the blood-vessels rim. When the 

 allantoic vesicle is flattened, these grooves necessarily produce notches 

 Ol' bays ill the margin of the \esicle, more or less deep according to 

 tlie size of the bl<x)d- vessels. In the case of the anterior set of the 

 allantoic \essels, the gi'oove luis bec(^me so deep that the right lobe 

 and tlie middle lohe on the two sides of it have met again and become 

 llrndy a])])ressed with each other, so that praeticallv these blood- 

 vessels are sup))orted in tlieir course by a mesentery- like i'old of the 

 allantoic \esicle. This explains the origin of the anterior constric- 

 tion ol' the allantois. In a similai' way the notch tliat divides the 

 larger part of tlie allantois into the right and left lobes is prodiice(l 

 bv the ])osterior Ol' lel't set of lilood-vessels, althongh th<' iiot<h is 

 not as deep as in the anterior constriction and consists ol two or tliree 

 minor indentations. 



The jiosterior constriction of tlie allantoic- \esicle given in fig, 

 B7 is also not very difficult to explain. There can be no d(»ubt tliat 

 it is due in the main to the fact that the vesiele finds itsell' iinalile 

 to spread freely over the eiubrvo on account of the sero-amniot ic 

 connection. The only thing it can do is to grow round tin; sero- 



