30 



YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



side of the gut and is roughly oval in section, with its long axis dorsoventral. It rapidly 

 decreases in size and ends at the sides of the cloaca at the level of the cloacal membrane. 



There is thus, just as Dandy (1910) and Wallin (1913) have described in younger 

 embryos, a continuous circuitous route through the body of the embryo from the exoccelom 

 of one side to that of the other, by way of the endoccelom, pleuroperitoneal passages, and 

 pericardial cavity. The question as to whether the cavities of the somites communicate 

 with the endoccelom is doubtful in many cases, owing to a slight maceration of the lateral 

 parts of the somites in the most favorable region for answering this. There is, however, in a 

 few cases in Embryo V, direct continuity of the cavity of the somite with the lumen of the 

 intermediate cell mass, which in turn opens through the nephrostome into the endoccelom. 



The septum transversum and ccelom of Embryo V are exactly similar in all respects to 



the above. 



SENSE ORGANS. 

 THE INTERNAL EAR. 



The only sense organ yet in evidence is the internal ear, excepting of course the optic 

 vesicles. No lens thickening and no olfactory placode are yet to be found. 



The internal ear is in the stage of the auditory cup, still widely open to the exterior. 

 It lies directly dorsal to the region of the second gill cleft. It is an egg-shaped invagination 

 of ectoderm, 3 to 4 layers of cells thick and surrounded, except on its medial side, by meso- 

 derm of the head. Medially it lies to the side of the hindbrain, and between it and the 

 brain is the hinder part of the acusticofacial ganglion, while the rest of the ganglion extends 

 out directly in front of it. The shape of the cavity throughout is rounded, with no indica- 

 tions of the approaching division into pars superior and inferior, already beginning to show 

 in Van den Broek's (1911) embryo, only 4 somites in advance of this in development. In 

 Low's (1908) embryo, 4 somites less than Embryo VI, there is only the slightest depression, 

 the thickened ectoderm mainly indicating this sen.se organ, and Wallin (1913) describes an 

 embryo of 13 somites with no evidence whatever of the invaginations. In Van den Broek's 

 embryo the opening from the outside is not equal in extent to the cup, but is much con- 

 stricted, being only 40m long, while the cephalocaudal diameter of the cup is 184/x. In 

 Embryo VI the opening comprises almost the whole cephalocaudal extent of the cup and so 

 even the extremities of the cup are not as yet constricted off from the ectoderm. The 

 auditory cups of Embryo VI are slightly larger, but those of Embryo V are entirely similar 

 in every way to the others. The main measurements are appended for both. These 

 measurements were not taken in the plane of the principal axes of the body of the embryo , 

 but in the plane of the principal axes of the auditory cup alone, which, lying on the sloping 

 body wall, has its axes oblique to those of the body. 



Outside diameters "I auditory cup. 



( lephalocaudal 

 Mediolateral 



Dorsoventral 



Diameters of opening into cup 



( !ephalocaudal 



Mediolateral 



The above measurements show how much alike in size and shape are these auditory 

 pits. Measurements given by Van den Broek for the embryos he describes, of 22 somites, 



