6(3 a HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



general amniotic cavity, is about 0.6 mm. The duct crosses the body-stalk almost 

 at right angles, close to the surface of the latter, and ends very near the distal 

 extremity of the allantois and close to the chorionic surface of the body-stalk a 

 short distance below the attachment of the stalk to the chorion. The lumen is 

 very irregular, at times wanting, and is lined by cells which vary from low cuboidal 

 to frankly cylindrical. Irregular thickenings or solid masses of cells also occur in 

 the walls, some of which seem to have almost lost their connection with the duct. 

 The whole structure is obviously undergoing retrogressive changes. Near the 

 summit of the amnion there occurs a second small, slightly branched, but quite 

 irregular diverticulum (not shown in the illustrations), which penetrates for some 

 little distance into the substance of the body-stalk in the region of its attachment to 

 the chorion. Tins slender, tubular structure is much smaller than the amniotic 

 duct mentioned above; it is also less sharply demarcated from the surrounding 

 mesenchyme, while its cells are rather more vacuolated. 



EMBRYO AND ECTODERM. 

 The external configuration of the embryonic body is determined in large meas- 

 ure by the stage of development of the nervous system, which is here in the form of a 

 groove, widely open throughout its entire extent. One may say that about one- 

 half of' this groove represents the future brain, and that at about the junction of 

 this with the cord, or even a little in advance of this point, is where the flexure of 

 the embryo is most marked. The primitive forebrain is especially prominent, being 

 represented by two conspicuous crescentic folds, highest and broadest behind and 

 approaching each other anteriorly, where they are separated by a shallow furrow. 

 Behind, these folds- are convex, separated by a deep groove, and the brain-wall here 

 is not especially thick. In front their median slopes are distinctly concave, passing 

 gently into the central depression of the neural groove. A similar, even more marked 

 concavity is found in this region in embryo No. 1201. It is at this point, in front, 

 that the walls of the neural groove are thickest, indicating the position of the later 

 optic pits. The median groove (the bottom of the neural groove) is continued over 

 the anterior surface of the head, spreading out and terminating in the buccopharyn- 

 geal" membrane just above the attachment of the somatopleure. This portion of the 

 brain, which we have identified as prosencephalon, is widest about its center, over- 

 hangs posteriorly the sides of the head ventral to it, and is placed at approximately 

 right angles to the remaining caudal part of the brain. 



Just behind the angle thus formed the brain is slightly narrower, but it soon 

 increases again in diameter to form a spindle-shaped enlargement, the posterior 

 limits of which are indistinct, This latter enlargement, which is not especially 

 conspicuous, is the rhombencephalon, while the faint constricted portion in front 

 of it we take to be the mesencephalon, although a part of what we have called fore- 

 brain, just at the angle, may later give rise to more or less of the mid-brain. In 

 other'words, the cephalic flexure is very well marked at this early stage. A better 

 term would be mid-brain flexure, since it is only the nervous system which is bent, 

 and this over the anterior, blind end of the fore-gut. A straightening out of the 



