THE EMBRYO. 



1189 



FIG. 745. Various forms of mother-cells under- 

 going- development into blood-vessels, from the 

 middle layer of the chick's blastoderm. (Klein.) 

 a. Large mother-cell vacuolated, forming the 

 rudimentary vessel. 6. The wall of this cell 

 formed of protoplasm, with nuclei embedded, and 

 in some cases more or less detached and projecting, 

 c. Processes connected with neighboring cells, 

 formed of the common cellular substance of the 

 germinal area. d. Blood-corpuscles. /. Small 

 mother-cells vacuolation commencing. B. 

 Mother-cell in which only obscure granular matter 

 is found. 



going multiplication by division resemble the white corpuscles. Soon, however, 

 true white corpuscles make their appearance, and it seems that they are derived 

 from the rudiments of the thymus gland. 1 The nucleated condition of the red 

 globules ceases before birth. The vitelline circulation commences about the 

 fifteenth day and lasts till the fifth week. When fully established, it is carried on 

 as follows : Proceeding from the anterior end of the tubular heart are two arteries, 

 the primitive aortae ; these run down in 

 front of the primitive vertebrae and behind 

 the walls of the intestinal cavity into the 

 two omphalo-mesenteric arteries, which 

 ramify in the vascular area of the yolk- 

 sac. Here they terminate peripherally in 

 a circular vessel the terminal simis, 

 which surrounds the vascular area. The 

 blood is collected from the capillaries of 

 the vascular area into the two omphalo- 

 mesenteric veins, which open into the pos- 

 terior extremity of the heart. 



2. The Placental Circulation. As the 

 umbilical vesicle diminishes, the allantois 

 and the placenta are developed in the 

 manner above described (page 1162). 

 When the umbilical vesicle atrophies the 

 placenta becomes the only source of nutri- 

 tion for the embryo. The allantois carries 

 with it two arteries (umbilical or allantoic), 

 derived from the primitive aortae, and two 

 veins; these vessels become much en- 

 larged as the placental circulation is established, but subsequently one of the veins 

 disappears, and in the later stages of uterine life the circulation is carried on 

 between the foetus and the placenta by two umbilical arteries and one umbilical 

 vein. 



During the occurrence of these changes great alterations take place in the 

 primitive heart and blood-vessels, and now require description. 



Further Development of the Heart. The following is an outline of the changes 

 which take place during the further development of the heart. 



The simple tubular heart, 

 already described, becomes elon- 

 gated and bent on itself, so as to 

 form an S-shaped loop, the anterior 

 part bending to the right and the 

 posterior part to the left. The 

 intermediate portion arches trans- 

 versely from right to left, and then 

 turns sharply forward into the 

 anterior part of the loop. Slight 

 constrictions make their appearance 

 in the tube and divide it into four 

 parts, viz. : (1) the sinus venosus 

 (sinus reuniens of His); (2) the 

 common auricle ; (3) the common 

 ventricle : (4) the aortic bulb. The 

 common auricle and ventricle communicate by a short canal, the auricular canal 



(Figs. 742, 743, and 746). 



The sinus venosus is situated in the septum transversum (a layer of mesoblasl 

 from which the ventral part of the Diaphragm is developed) behind the . 

 1 Consult an article by J. Beard, Anatomischer Anzeiger, December, 1900. 



Bifiht 

 auricle. 



Auricular . 

 canal. 



,.- Left 

 auricle. 



-Ventricle. 



FIG 746, Heart of a human embryo of 5 mm. in length, seen 

 from 'the front; X 30. (His.) 



