696 GENERAL PHEXOMEXA OF EMBP.YOXIC DEVELOPMENT. 



epiblast, and other ocular structures proceed from the mesoblast. In 

 the ear the labj'rinth originates by invohition of its cavity from the 

 epiblast in the neighbourhood of the third encephalic vesicle, and the 

 auditory nerve growing out from the medullary wall of the third 



Fig. 507. — Section of the Com- 

 mencing Eve of an Embryo 

 in thkee stages. 



A. Commencement of the for- 

 niation of the lens by dei^ression 

 of a part of C, the corneous layer ; 

 2^r, the jjrimitive ocular vesicle 

 now doubled back on itself by the 

 depression of the commencing 

 lens. 



15. The lens depression enclosed 

 and the lens beginning to be 

 formed in the inner side, the optic ve.ssel more folded back. 



C. A third stage in which the secondary optic vesicle, r, begins to be formed. 



encephalic vesicle, is subsequently extended into the ear vesicle ; ■while 

 the middle and outer ear cavities are developed from mesoblastic ele- 

 ments in connection with the first and second post-oral subcranial plates 

 and the intervening pharyngeal cleft. In the nose likewise the open 

 cavity afterwards occupied by the distributed extremities of the olfactory 

 nerves originates by depression or involution from the epiblast in front 

 of the first encephalic vesicle of the cranium. 



Vascular system. — The next important series of changes by which 

 the foundations of the great organic systems are laid consists in the 

 formation of the rudiments of the heart, blood-vessels and blood, 

 and in the establishment of the first circulation. The several part^ of 

 the sanguiferous system all originate in the deeper or splanchnopleural 

 division of the mesoblast, but once formed in this section of the 

 blastoderm the blood-vessels very soon extend into all other parts which 

 are vascular. 



Fig. 508. 



as soon as the blastodermic cells „ 

 first sunple tubular form of the organ. 



Fig. 508. — Outlines of the anterior 



HALF OF THE EmbRYO ChICK VIEWED 

 FROM BELOW, SHOWING THE HeART IN 

 ITS EARLIER STAGES OF FORMATION 



(after Remak). "^ 



A, Embryo of about 28 to SO hours ; 

 B, of about 36 to 40 hours ; a, anterior 

 'cerebral vesicle ; h, proto-vertebral seg- 

 ments ; c, cephalic fold ; 1, 1, primi- 

 tive omf)halo-mesenteric veins entering 

 the heart posteriorly ; 2, their union n 

 the auricle of the heart ; 3, the middle 

 l^art of the tube corresponding to the 

 ventricle ; 4 (in B) the arterial bulb. 



The formation of the heart, 

 blood-vessels and blood is nearly 

 simultaneous, and the rhythmic 

 contractions of the heart begin 

 have arranged themselves in the 



