576 SUMMARY OF CUERENT RESEARCHES RELATING TO 



In dealing with the primitive groove, he reaffirms the statement 

 that it is a median thickening of the ectoderm, and that the endoderm 

 takes no share in forming it. This groove soon begins to shorten at 

 both ends, and becomes thicker at the anterior, and thinner at the 

 posterior end. 



The medullary tube has a double mode of origin ; the greater 

 part arises by the formation of a groove in the ectoderm, but the 

 hinder portion is at first solid (medullary cord). In its anterior 

 part, and in front of the primitive groove, it is pm-ely ectodermal in 

 origin ; in the region of the groove the mesoderm takes a part in 

 the formation of its floor ; behind this, it is solely of mesodermal 

 origin. As to the difficult question of how this hinder part is formed, 

 the author points out that there are three possible ways. The cord 

 may have grown from the point where there is a connection between 

 the medullary tube and the ectoderm backwards into the thickened 

 portion of the primitive groove ; or there may have been no con- 

 nection with the ectoderm ; or, thirdly, the mesodermal elements 

 may only secondarily have entered into its composition. To this last 

 view the author would ai)pear to incline. 



The chorda dorsalis has already been shown by Braun to be deve- 

 loped from the axial part of the mesoderm. So far as the anterior 

 part of the primitive groove extends, the chorda arises from it, and 

 there is an intimate connection between the dorsal surface of the 

 notochord and the future ventral limit of the medullary tube. In 

 many cases the deeper axial cells of the primitive groove collect 

 in the middle line, and become bounded off laterally, and this sepa- 

 ration takes place from below upwards. In other cases the chorda 

 dorsalis becomes marked off more anteriorly, at the point where the 

 ectoderm is already separated in the middle line from the mesoderm, 

 and here, of course, it appears to be altogether mesodermal in 

 origin. 



The communication between the dorsal medulla and the endo- 

 derm, which was first seen by Gasser,* has been further extended by 

 Braun, who has detected two other communicating orifices ; he here 

 discusses his results, and compares them with what has been described 

 by other observers, but he refuses to follow some of them into the 

 formation of impossible hypotheses. 



His investigation into the development of the caudal end of the body 

 has led him to the interesting result that, after the formation of the 

 rudiment of the tail, there is a communication at its hinder end 

 between organs so different in function and developmental history as 

 the dorsal medulla and the caudal enteron. 



Structure of the Mammalian Ovary.f — In a second essay on 

 this subject, Mr. Jules Macleod deals with the orang-outang, with 

 Semnopithecus, Cercopithecus, Macacus, and Cynocephalus, and with one 

 lemur L. nigrifrons. 



Among the Primates, the human species is the only one in 



* See this Journal, ante, p. 217. 



t Arch, de Biol., ii. (1881) pp. 127-44 (2 pis.). 



