TIM-: POETA1 MI-:MI:K.\\I> 01 I AMI 1.1 _ \'t 



Fig. 4. Liinyitudinal section tlinnnjh a ll< ,,'s ,-,/,/ at tin- h<,j'<nn\n<j of t ht 



After tin- fii-ioii nf tin- amnii.: U \\ilh the ami 



ca\it\ (aA), is enveloped in the amniotic sao, Tin- serous mem!.]. l>een 



develoj ed from the outer layer of the amniotic folds. r,\ further >rj.aration 

 of tin- iniilillt- Lfenr.-layers the extra-embryonic ] art f the body-cavit y (///-') 

 has enlarged, ami tlie allantois \al \ has L r n>\\n into it. 



With tlie exception of a thinl >f it.- Mirface. the yolk lias become overgrown 

 by th,- miter an. 1 inner irerm-layer-. as t. line tW, The va>cul:.r 



has xtended to the line st. The cephalic portion <>: v has 



unniotic cavity by means of tin- ~<-n mouth (m). 



~>. -- Loni/it tnllinil sect 'nut throi/t//t n llrii'x t'tit/ mi thf xt'i'cntli iltuj of 



By the enlargement of the extra-embryonic. 1> niy-eavity the serous membrane 

 (serosa^ has entirely sejiarated from the \olk-snc, with the exception of a small 

 ana. 1 he oilier and the inner germ-layers have now grown over the yolk on all 

 : the middle germ-layer with the vascular area has extended fa r. her 

 downwards. The amniotic cavity, in which the embryo floats, has become 

 much extended by the increase of the amniotic fluid. The allantois has 

 enlarged considerably, and forms a sac, which connects with the hind irut 

 by means of a narrow stalk (urachus). The sac extends out into the extra- 

 embryonic body-cavity between amnion, yolk-sac, and serous membrane, more 

 particularly on the right side of the embryo. 



Fiir. t> rrjnrxi'itts a iliti</r<i>n>n<itic cruns st'ctiot through an embryo 



The dorsal part is already far advanced in development and encloses the 

 neural tube (JV), the chorda (cA), the aorta (ao), and the primitive segments. 

 The ventral side is greatly distended by the considerable yolk-mass (rf). The 

 latter lies in an enlargement of the intestinal canal, the intestinal yolk 

 this is separated from the enlarged dermal yolk-sac by means of a narrow 

 fissure, the body-cavity (Ik). 



7. I)i<iijrninm(i1ic lonr/itiuliiutl st -rt'ton through < * n embryo. 



The yolk-sac has been partly constricted off from the body of the emoryo ; 



it still remains united to its ventral side, but only by nn-ansof a narrow >ta' 

 which consists of two tubes, one within the other, the intestinal >talk (vitelline 

 duct) and the dermal stalk. The yolk-sac communicates with the embryonic 

 intestinal canal by means of the vitcllinc duct. Tlie point of transition is 

 called the intestinal umbilicus (dii). The point of attachment of the dermal 

 stalk to the belly of the embryo is the dermal umbilicus (fin). The space 

 between dermal and intestinal umbilicus (hn and <i to put the body 



cavity of the embryo (Ik 1 ) in communication with the body-space (//r) between 

 the dermal and intestinal yolk-sacs. 



