The development of the human body; a manual of human embryology . medullary groove; and in the older embryo described bySpee (Fig. 38) a longitudinal ridge has appeared on eitherside of the groove, forming the medullary folds. The two folds are continuous anteriorly, but behindthey are at first separate, the anterior portion of the primi-tive streak lying between them. In forms, such as theReptilia, which possess a distinct blastopore, this openinglies in the interval between the two, and consequently is inthe floor of the medullary groove, and in the mammalia,even though no well-defined blast
The development of the human body; a manual of human embryology . medullary groove; and in the older embryo described bySpee (Fig. 38) a longitudinal ridge has appeared on eitherside of the groove, forming the medullary folds. The two folds are continuous anteriorly, but behindthey are at first separate, the anterior portion of the primi-tive streak lying between them. In forms, such as theReptilia, which possess a distinct blastopore, this openinglies in the interval between the two, and consequently is inthe floor of the medullary groove, and in the mammalia,even though no well-defined blastopore is formed, yet atthe time of the formation of the medullary fold an openingbreaks through at the anterior end of the primitive streakand places the cavity lying below the endoderm in com-munication with the space bounded by the medullaryfolds. The canal so formed is termed the neurentericcanal (Fig. 55, nc) and is so called because it unites whatwill later become the central canal of the nervous systemwith the intestine (enteron). The significance of this. Fig. 54.—Embryo mm. , Allantois; Am, amnion; bs, belly-stalk; h, heart; m, medullary groove;, neurenteric canal; , caudal protuberance; ps, primitivestreak; ys, yolk-stalk.—(Etcrnod.) 10 113 H4 THE DEVELOPMENT OF THE HUMAN BODY. canal is somewhat obscure, and it is of very brief persist-ence, closing at an early stage of development so as toleave no trace of its existence. As development proceeds the medullary folds increasein height and at the same time incline toward one another
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
