166 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



dark and irregular, and considerable fibrin is present. The so-called "cell columns " 

 (Grosser) are similar masses of cells, by means of which the villi are attached to the 

 decidua. They are numerous at the ends of the villi. The intervillous spaces are 

 remarkably free from maternal blood-corpuscles, only a few being found. 



The stroma of the chorion and its villi is made up of a network of loose mesen- 

 chyma. Its cells are stellate in shape and finely granular; the nuclei are oval, 

 rich in chromatin, and contain nucleoli. In places are to be found greatly elon- 

 gated nuclei, much resembling those of smooth muscle-fibers. These are found 

 just beneath the Langhan's layer. 



Throughout the interspaces of the mesenchyma are to be found blood-vessels 

 of various sizes, filled in many instances with blood elements; also the so-called 

 "Hofbauer" cells (Grosser 13 ) with highly vacuolated protoplasm and large nuclei; 

 these appear in the body-stalk as well and occasionally in blood-vessels. 



EXPLANATION OF PLATES. 



PLATE 1. 



1. Model of whole embryo viewed from left side. The body-wall in the region of the heart and septum transversum 



has been dissected away. At X, pericardo-pleural passage. X80 diameters. 



2. Same, viewed from in front. X80 diameters. 



PLATE 2. 



1. Brain and portion of neural tube. In light numbers are indicated brain and spinal-cord neuromeres, numbered 



from the mesencephalon backward. The black lines and numbers indicate the position of the body-seg- 

 ments. It is to be noted that the neuromeres are intersegmental in position. X80 diameters. 



2. Pharynx, cut longitudinally and left half viewed from medial surface. X133 diameters. From a model by 



Mr. L. H. Rutledge. 



3. Same, viewed from right side with integument removed. X 133 diameters. 



4. Ventral surface of same. X 133 diameters. 



PLATE 3. 



1 . Heart as seen from dorsal surface. From a model by Mr. Florien Vaughn. X 166 diameters. 



2. Cavity of the heart as seen from dorsal surface. From a model by Mr. Florien Vaughn. X166 diameters. 



3. Model of fore-gut viewed from left side. X266 diameters. 



4. Ventral surface of same. X266 diameters. 



PLATE 4. 



1. Portion of section 94 showing beginning of heart sinusoid in ventricular portion of heart. X600 diameters 



2. Portion of section 165 passing through twelfth somite. X325 diameters. 



3. Portion of section 116. X600 diameters. 



4. Xephric system of left side viewed from medial surface. From wax reconstruction. X125 diameters. 



PLATE 5. 



1. Graphic reconstruction of venous system. X80 diameters. At X, junction of large veins with sinus venosus. 



2. Graphic reconstruction of arterial system. X80 diameters. Between X and X the dorsal aortae have fused, 



forming a single vessel. 



PLATE 6. 



1. The sinus venosus and entering veins. From a wax reconstruction by Mr. T. F. Wheeldon. X120 diameters. 



2. Wax reconstruction showing relations of the dorsal segmental arteries. Viewed from behind forward. X120 



diameters. 



3. Wax reconstruction of the ccclomic cavity viewed from in front. X63 diameters. 



4. Same, viewed from behind. X63 diameters. 



PLATE 7. 



1. Portion of section 127, showing ccelomic depressions in region of septum transversum and amnion. X400 



diameters. 



2. Portion of section 269, showing ccelomic depressions in region of body-stalk. X275 diameters. 



3. Cast of ccelomic depressions in region of transverse septum. The irregular depressions are in places branched, 



as shown on *,he left side of figure. X215 diameters. 



4. Villus from chorion frondosum. From a wax reconstruction by Mr. H. L. Houchins. X60 diameters. 



5. Graphic reconstruction of left half of embryo, viewed from medial side. Numerous models have been used in 



making the drawing. X60 diameters. 



PLATE 8. 

 1-12. Cross sections of embryo. X40 diameters. 



1. Section 31. 4. Section 95. 7. Section 140. 10. Section 211. 



2. Section 48. 5. Section 110. 8. Section 162. 11. Section 219. 



3. Section 63. 6. Section 123. 9. Section 180. 12. Section 254. 



