fBRYOLC 



in the body-wall toward the ventral median line, thus separating the pericardium 

 from the lateral thoracic walls (Fig. 53) . The further development of the peritoneal 

 cavity has been described with the development of the digestive tube (page 168 

 et seq.}. 



Spleen 

 Colon 



Suprarenal gland 

 Eleventh rib 

 Twelfth rib 



Stemo-costal part of 



Diaphragma 

 Central tendon of Diaphragma 



Inferior vena cava 



(Esophagus 



Vertebral part of Diaphragma 



Posterior mediastinal cavity 



Aorta 



Spino-costal hiatus 



Left pleura 



Sight pleura 



FIG. 57. The thoracic aspect of the diaphragm of a newly born child in which the communication between the 

 peritoneum and pleura has not been closed on the left side ; the position of the opening is marked on the right side by 

 the spinocostal hiatus. (After Keith.) 



THE FORM OF THE EMBRYO AT DIFFERENT STAGES OF ITS GROWTH. 



First Week. During this period the ovum is in the uterine tube. Having been fertilized in 

 the upper part of the tube, it slowly passes down, undergoing segmentation, and reaches the 

 uterus. Peters 1 described a specimen, the age of which he reckoned as from three to four days. 

 It was imbedded in the decidua on the posterior wall of the uterus and enveloped by a decidua 

 capsularis, the central part of which, however, consisted merely of a layer of fibrin. The ovum 

 was in the form of a sac, the outer wall of which consisted of a layer of trophoblast; inside this 



Heart 



Amnion 



Body-stalk 



Chorion 

 FIG. 58. Human embryo about fifteen days old. (His.) 



was a thin layer of mesoderm composed of round, oval, and spindle-shaped cells. Numerous 

 villous processes some consisting of trophoblast only, others possessing a core of mesoderm 

 projected from the surface of the ovum into the surrounding decidua. Inside this sac the rudi- 

 ment of the embryo was found in the form of a patch of ectoderm, covered by a small but com- 



1 Die Einbettung des menschlichen Eies, 1899. 



