FORMATIONS FROM THE EPIBLAST. 



967 



FIG. 376. I, The three germinal layers of the ovum of mammals: Z, Zona pellucida; E, epiblast; m, mesoblast, 

 e, hypoblast. II, Cross-section of chick (with six. primitive vertebra?) on the first day: M, spinal furrow, h, 

 epidermis; U, primitive vertebra; c, chorda dorsalis; S, the lateral plates divided into two lamellae; e, hypo- 

 blast. Ill, Cross-section of chick on the second day, in the region behind the heart: M, medullary canal; 

 h, epidermis; u, primitive vertebra; c, chorda; w, Wolffian duct; K, ccelom; x, cutaneous plate; y, splanch- 

 nopleura; A, amniotic fold; a, aorta; e, hypoblast. IV, Diagrammatic representation of the first embryonal 

 rudiment in longitudinal section. V, Diagrammatic representation of the beginning of the process of con- 

 striction: r, headfold; D, cavity of the foregut; S, caudal fold; d, hind-gut cavity in an early stage of forma- 

 tion. VI, I >kiKrammatic longitudinal section of the embryo after constriction: A o, omphalomesaraic artery; 

 V o, omphalomesaraic vein; a, rudimentary allantois; A, amniotic fold. VII, Diagrammatic longitudinal 

 section through a human ovum: Z, zona pellucida; S, serous capsule; r, union of amniotic folds; A, amniotic 

 cavity; a, allantois; N, umbilical vesicle; m, mesoblast; h, heart; U, primitive gut. VIII, Diagrammatic 

 longitudinal section through the pregnant uterus at the time of the formation of the placenta: U. muscular 

 wall of the uterus; p, mucous membrane of the same or true decidua; b, maternal placenta or serotine decidua; 

 r, reflex decidua; ch, chorion; A, amnion; n, umbilical cord; a, allantois with urachus; N, umbilical vesicle 

 with D, the omphalomesaraic duct; t, t, tubal openings; G, cervical canal. IX, Human embryo at the time 

 of the visceral arches (diagrammatic) : A, amnion; V, forebrain; M, midbrain; H, hind brain; N, afterbrain; 

 U, primitive vertebra; a, eye; p, nasal depression; S, Frontal process; y, internal nasal process; n, external 

 nasal process; r, superior maxillary process of the first visceral arch; i, 2, 3, 4, the four visceral arches with 

 the intervening clefts; o, auditory vesicle; h, heart with, e, the primitive aorta, which divides into the five 

 aortic arches; f, descending aorta; om, omphalomesaraic artery; b, the same artery upon the umbilical vesicle, 

 B; c, omphalomesaraic vein; L, liver with the venae advehentes and revehentes; D, gut; i, inferior cava; 

 T, coccyx; all, allantois with, z, an umbilical artery, and, x, an umbilical vein. 



