STRUCTURES FORMED FROM THE MESOBLAST. 



867 



V o, omphalomesenteric veins ; a, position of the allantois ; A, amniotic fold. VII, 

 Scheme of a longitudinal section through a human ovum Z, zona pellucida ; S, serous 

 cavity ; r, union of the amniotic folds ; A, cavity of the amnion ; a, allantois ; N", um- 

 bilical vesicle ; ra, mesoblast ; h, heart ; U, primitive intestine. VIII, Schematic trans- 

 verse section of the pregnant uterus during the formation of the placenta ; U, muscular 

 wall of the uterus ; p, uterine mucous membrane, or decidua vera ; b, maternal part of the 

 placenta, or decidua serotina ; r, decidua reflexa ; ch, chorion ; A, amnion ; n, umbilicai 

 cord ; a, allantois, with the urachus ; N, umbilical vesicle, with D, the omphalo-inesen- 

 teric duct ; 1 t, openings of the Fallopian tubes ; G, canal of the cervix uteri. IX, Scheme 

 of a human embryo, with the visceral arches still persistent A, amnion ; V, fore-brain ; 

 M, mid-brain ; H, hind-brain ; N, after-brain ; U, primitive vertebrae ; a, eye ; p, nasal 

 pits ; S, frontal process ; y, internal nasal process ; n, external nasal process ; r, superior 

 maxillary process of the 1st visceral arch ; 1, 2, 3, and 4, the four visceral arches, with the 

 visceral clefts between them ; 0, auditory vesicle ; h, heart, with e, primitive aorta, which 

 divides into five aortic arches ; /, descending aorta ; om, omphalo -mesenteric artery ; b, the 

 omphalo-mesenteric arteries on the umbilical vesicle ; c, omphalo-mesenteric vein ; L, liver, 

 with vense advehentes and revehentes ; D, intestine ; i, inferior cava ; T, coccyx ; all, 

 allantois, with z, one umbilical artery, and x, an umbilical vein. 



over the other. The upper layer or epiblast is the larger, and contains small pale cells ; the 

 lower layer, or hypoblast, which at first is not a continuous layer, ultimately forms a continuous 

 layer, but its periphery is smaller than 

 the upper layer, while its cells are 

 larger and more granular. 



Between the epiblast and hypoblast 

 there is formed, from the primitive 

 streak as a product of cell-proliferation , 

 the mesoblast, which is said by Kol- 

 liker to be due to the division of the 

 cells of the epiblast. It gradually ex- 

 tends in a peripheral direction between 

 the two other layers. All the three 

 layers grow at their periphery. In 

 the mesoblast blood-vessels are de- 

 veloped. All the three layers, as they 

 grow, come ultimately to enclose the 

 yelk, so that their margins come to- 

 gether at the opposite pole of the yelk. 



441. STRUCTURES FORM- 

 ED FROM THE MESOBLAST 

 AND HYPOBLAST. Themeso- 

 blast (vascular layer or middle 

 layer) forms immediately under 

 the medullary groove, a cylindri- 

 cal cellular cord, the chorda dor- 

 salis, or notochord, which is 

 thicker at the tail than at the 

 cephalic end (fig. 665, II, III, c). 

 It is present in all vertebrata, and 

 also in the larval form of the 

 ascidians, but in the latter it dis- 

 appears in the adult form (Kowa- 



lewsky). In man it is relatively P , , , -^ , V 8 ' DD : 

 11 Ti. t iu u -1 Embryo fowl of the 2nd day, x 50. 



small. t forms the basis of the area y pel i ucida . m , hind-brain ; Mh, mid-brain ; Vic, 



fore-brain ; om, omphalo-mesenteric veins ; omr, point 

 where the closure of the neural groove is travelling 

 backwards with the protovertebrse ; Vw, muscle-plates ; 

 Rf, posterior part of widely-open neural groove ; Rw, 

 neural ridge ; vAf, anterior amniotic fold. 



Fig. 666. 



Ao, area opaca ; Ap, 



bodies of the vertebrae, and around 



it, as a central core, the substance 



of the bodies of the vertebrae is 



deposited, so that they are strung 



on it, as it were, like beads on a 



string. After it is formed, it becomes surrounded by a double sheath-like covering 



(Gegenbaur, Kolliker). 



