280 SUMMARY" OF CURRENT RESEARCHES RELATING TO 



endoderin. Beyond the sinus the yolk-sac endoderm is intimately 

 connected with the trophoblast. At the middle of the third week the 

 splitting of the mesoderm is only beginning, and the amnion is only 

 represented by indistinct lateral ridges. At the end of the third week 

 the amnion is complete, and the mesoderm is split so as to form a 

 coelom and a small exocoelom. There is no indication of an allantoic 

 diverticulum at the middle of the third Aveek, but it has appeared at 

 the end. 



The 21 -day embryo has over twenty somites, and is hook-shaped. 

 The nervous system is represented hj a fore-brain with optic vesicles, 

 by an indistinct mid-brain, by a hind-brain adjacent to which are otic 

 vesicles, and by a spinal cord completely closed except at the caudal 

 end, where it opens by a neuropore into the cavity of the amnion. The 

 alimentary system consists (1) of a fore-gut from which branchial pouches 

 project outwards towards shallow branchial farrows lying between 

 branchial arches, (2) of a mid-gut which communicates by a yolk-stalk 

 with a large yolk-sac, and (3) of a short hind-gut from which a cloacal 

 rudiment projects upwards, and an allantoic diverticulum extends back- 

 wards. The circulatory system is represented by a sinus venosus, an 

 atrium, a ventricle and a bulbus arteriosus ; by two pairs of aortic 

 arches, two dorsal aortfe, by vitelline and other arteries, and by vitelline, 

 cardinal, umbilical and other veins. A notochord and branchial arches 

 represent a skeleton, and excretory organs are represented by Wolffian 

 bodies and pronephric ducts. 



In the comparison between horse and sheep embryos it is pointed 

 out that even at the stage characterized by four pairs of mesodermic 

 somites the horse differs from a sheep at the corresponding phase in its 

 life-history, and that, as development proceeds, tlie differences, more 

 especially in the foetal appendages, l)ecome more pronounced. 



Derivatives of Human Pharynx.* — B. F. Kingsbury considers the 

 pharynx as consisting morphologically of four zones : the hypobranchial, 

 the floor, the epi- or hyper-branchial, and the lateral wall or branchial 

 region proper. The investigation of the growth shiftings in the domain 

 of the pharynx has to deal with three parts : (1) that of the dorsal 

 wall and cephalic portion, including the first (and second) branchial 

 arches and the first and second pouches ; (2) that of the tongue and 

 the corresponding portions of the pharyngeal floor ; (3) the caudal and 

 ventral portions, including the third and fourth pouches and clefts 

 and the second cleft, together with the thyroid gland. It is mainly the 

 last section of the pharynx development that is considered in the present 

 paper. 



In a 3 mm. embryo the thyroid gland is present as a median out- 

 pocketing, still broadly in communication with the caudal limb of the 

 first branchial pouch and in immediate contact with the truncus aorticus. • 

 Its history and that of the third complex is followed, and the effect of 

 the descent of the heart is analyzed. The epibranchial placodes, the 

 thymus, the parathyroids, and ultimobranchial body are then discussed. 



Kingsbury passes to a consideration of endocrine or internally 



* Amer. Joum. Anat., xviii. (1915) pp. 329-97 (5 pis.). 



