AND THE FOETAL MEMBRANES, IN THE CHICK. 347 



in both of which the epiblast aud the mesoljlast are by this time 

 inseparably attached, are stil] comiected along the line corresponding 

 to the length of the sero-anmiotic connection hy means of insinuated 

 mesoblastic net-W(jrk. Thus the mewhlaüic or secondanj >iero-am.niotic 

 connection is formed. 



6. Expan,s((jn of the Mesoblastic Connection. 



The siin[)Ie replacement of the epil)Iastic Ijridge by the mesoblastic 

 net- work is not the sole event in the formation of the secondary sero- 

 amniotic connection. From the nature of the mesoblastic connection 

 its breadth is of course invariably greater than that of the previous 

 epiblastic connection. But, (jver and beyond this, the mesoblast of 

 the amnion and that of the serous envelope begins after the first week 

 of the incubation to coalesce for a certain distance on both sides of the 

 connection, so that in ten to twelve days, the connection is very much 

 widened and assumes the form of a plate, viewed from above. Fi^-s. 

 4o-50 represent the l^readth (jf the connection at different stao-es, eadi 

 projected on a straight line. Figs. 7G, 81, and 84 are transverse 

 sections of the secondary connection, through the approximately middle 

 level of the connection represented in Figs. 45, 4i), and 50 respe<-- 

 tively. 



If we examine sections of embryos in tlieir seventh to tenth dav of 

 the incubation, the ])rocess of coalescence is unfailingly witnessed. The 

 plate-like mesoblastic connection seems to be produced by the intimate 

 contact of mesoblastic cells (Fig. 75) in the narrow space on both 

 sides of the preexisting connection, similar to the coalescence of the 

 allantois with the serous envelope (Section 4). 



The breadth (jf the secondary connection may, however, vary to 

 a certain extent in different individuals, and not necessarilv be pi'o- 

 })ortional to the age of the embryo (Figs. 4o-5()). 



