

THE BRANCHIAL REGION 



65 





through its thickness and are attached to the chorion. In the early months these 

 septa convey branches of the uterine arteries which open into the intervillous 

 space on the surfaces of the septa. The fetal surface of the placenta is smooth, 

 being closely invested by the amnion. Seen through the latter, the chorion 

 presents a mottled appearance, consisting of gray, purple, or yellowish areas. 

 The umbilical cord is usually attached near the center of the placenta, but 

 may be inserted anywhere between the center and the margin; in some cases it 

 is inserted into the membranes, i. e., the velamentous insertion. From the attach- 

 ment of the cord the larger branches of the umbilical vessels radiate under the 

 amnion, the veins being deeper and larger than the arteries. The remains of 

 the vitelline duct and yolk-sac may be sometimes observed beneath the amnion, 

 close to the cord, the former as an attenuated thread, the latter as a minute sac. 

 On section, the placenta presents a soft, spongy appearance, caused by the 

 greatly branched villi; surrounding them is a varying amount of maternal blood 

 giving the dark red color to the placenta. Many of the larger villi extend from 

 the chorionic to the decidual surface, while others are attached to the septa which 

 separate the cotyledons ; but the great majority of the villi hang free in the inter- 

 villous space. 



Mid-brain 



Fore-brain 

 Stomodeum 

 andibular arch 

 Heart 



Eye 



Hind-brain 



Auditory vesicle 



Visceral 

 arches 



Olfactory pit 

 Maxillary process 

 Amnion (cut) M andibular arch 



Hyoid arch 

 Third arch 



Body-stalk 



Fio. 40. Embryo between eighteen and twenty-one 

 days. (His.) 



FIG. 41. Head end of human embryo, about the end 

 of the fourth week. (From model by Peter.) 



THE BRANCHIAL REGION. 



The Branchial or Visceral Arches and Pharyngeal Pouches. In the lateral walls 

 of the anterior part of the fore-gut five pharyngeal pouches appear (Fig. 42) ; each 

 of the upper four pouches is prolonged into a dorsal and a ventral diverticulum. 

 Over these pouches corresponding indentations of the ectoderm occur, forming what 

 are known as the branchial or outer pharyngeal grooves. The intervening mesoderm 

 is pressed aside and the ectoderm comes for a time into contact with the ento- 

 dermal lining of the fore-gut, and the two layers unite along the floors of the 

 grooves to form thin closing membranes between the fore-gut and the exterior. 

 Later the mesoderm again penetrates between the entoderm and the ectoderm. 

 In gill-bearing animals the closing membranes disappear, and the grooves become 

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