THE THIRTEENTH AND FOTJETEENTH DAYS. 



481 



places the mid-gut and yolk-sac in direct communication with, 

 the exterior. It will be noticed that the wall of the yolk-sac 

 consists of an inner lining of hypoblast, H, and an outer wall 

 of mesoblast, mh, in which are very numerous blood-vessels. 

 On the right side the mesoblast was torn in the section, and is 

 indicated by a dotted line in the figure. 



The amnion, ax, in all three figiu-es is seen to consist of both 

 epiblastic and mesoblastic layers. 



Zollmann's embryo (Fig. 185) is rather older than the 

 others described in this section, and may be estimated as of the 

 fourteenth day. It affords an important transitional stage 



AM MX ^G 



W 



Fig. 185. — Human Embryo of about the fourteenth day, from the right side. 

 The yolk-sac and the wall of the blastodermic vesicle have been removed 

 (After KoUmann.) x 27. 



Alf, outer or true amnion. BF, fore-brain. DS, stomatodaeum. MT, mesoblastic 

 somite or protovertebra. NG, neural groove. ISG', point behind which the neural 

 groove is closed to form the neural tube. R, heart. TL, tail. TZ, allantoic stalk. 

 "W, vitelline vein. YS, yolk-stalt, cue short. 



between the embryos E and SR on the one hand, and on the 

 other the embryos of the third week, which will be described in 

 the nest section. 



Kollmann's embryo (Fig. 185) measures 2-5 mm. in length. 

 As compared with the earlier embryos, the head is larger and 

 more prominent; and the embryo is much more distinctly 

 constricted off from the yolk-sac. The neural folds have met 

 and fused, to complete the neural canal, in the hinder part of 

 the body, but the neural groove is stUl widely open in the head 

 and anterior part of the body. The bi-ain vesicles are becoming 

 evident, and the flexure of the anterior end of the head is already 



II 



