YOUNG TWIN HUMAN EMBRYOS WITH 17-10 PAIRED SOMITES. 13 



4.9 mm. long. He found thickenings over 4 arches, 4 gill pouches being present in his 

 embryo; only 2 gill pouches are to be found here, yet thickenings are present over 3 arches. 

 There is as yet no trace whatever of the formation of the lens of the eye or of the olfac- 

 tory placode. In the condition of the cloacal membrane this embryo agrees with that 

 described by Mrs. Gage (1905), who found the anal plate thickened in an embryo of 28 

 somites. Ingalls found it very thin and in places almost unrecognizable in one of 35 

 somites. In the present embryo, at the 18-19 somite stage, it is quite thickened and 

 differs in appearance from the adjacent ectoderm. The endoderm, as in the case of the two 

 embryos cited above, is in contact with and apparently fused to the ectoderm, and is also 

 thickened. 



MESODERMIC SOMITES. 



These embryos stand mid-way between stages 5 and 6 in Keibel and Elze's Normen- 

 tafel, where the embryos possess 13-14 somites, and stages 7 and 8, of 23 paired somites. 

 Embryo VI possesses 18-19 paired somites. There are 18 fully formed and distinct pairs 

 of somites and a nineteenth pair is well formed but not yet cut off from the unsegmented 

 mesoderm behind (text fig. 3). The first somite lies at the level of the posterior portion 

 of the heart. It is a peculiar coincidence that the last somite occurs exactly at the level 

 of the opening of the medullary canal into the medullary groove at the posterior neuropore. 

 This also is the exact level of the fusion posteriorly of the somatopleure and splanchnopleure, 

 cutting off ventral communication between the endoccelom and the exoccelom. 



The general shape of the somites is that of a triangular prism with rounded anterior 

 and posterior ends, and a ventral, a medial, and a dorsolateral wall. The more posterior 

 somites are more quadrangular than prismatic. The fourth, fifth, and sixth somites lie 

 somewhat obliquely to the long axis of the body, and between their posterior ends and the 

 medullary canal the anterior ends of the succeeding somites are inserted. The eleventh 

 to fifteenth pairs of somites inclusive lie in the region of the concave dorsal bend and the 

 posterior portion of the dorsolateral surface of each of these is overlapped by the anterior 

 end of the next. 



In describing the various parts of the somite, dermatome is the term used to denote 

 the flat plate forming the whole dorsolateral surface, myotome denotes the dorsal half of 

 the medial surface, and sclerotome the lower half of the medial surface, the whole of the 

 ventral surface, and the core of the somite. 



The first somite is very small. Its ventromedial portion is of somewhat looser, more 

 irregular tissue than the rest of the somite and suggests the beginning differentiation of the 

 sclerotome. There is" a distinct solid core fused with this and almost obliterating the cavity 

 of the somite, which appears only as a narrow slit. The dermatome and myotome are 

 differentiated from each other only by the presence of the upper myotomic groove. The 

 somite lies surrounded on all sides by dense mesenchyme with which it fuses anteriorly, so 

 that there is no distinct anterior end to it. This fusion of mesenchyme and first somite 

 is exactly similar to the condition described for the chick by Williams (1910), who found 

 in a series of embryos that this somite is the first formed both in time and position, but 

 develops extremely slowly compared to the others. The second somite shows the greatest 

 degree of development of the whole series and each succeeding somite shows a retrogressive 

 decrease in the degree to which it has attained. Each somite shows not only differences in 

 degree of development but also individual differences in shape and appearance, so that no 

 two exactly correspond. The second somite in this embryo shows almost exactly the 



