882 



„closure" of the neural plate we had a closed plate which after- 

 wards opened to let out the ganglionic masses on each side. This 

 is not the case. The ganglionic Anlagen really arise much 

 earlier, and while the plate is still quite open. Here, as in the Chick, 

 they are first evident in embryos in which the first mesoblastic 

 somites are being formed. On each side of the open neural plate and 

 just outside its limits a triangular wedge shaped mass of cells is seen 

 in section to be cut out of the inner portion of the thickened epiblast. 

 It gets separated from the epiblast above and beyond it, and gets 

 carried up during the involution of the neural plate along with the 

 latter. — Later on, when the involution is complete, and the epiblast 

 has closed over the neural plate, the Anlagen form paired masses of 

 cells lying between the lips of the neural plate and preventing its 

 closure. 



At this stage the epiblast just above the neural plate is only one 

 layer thick. Laterally the one layered epiblast passes into a much 

 thickei' triangular portion which is the Zwischenstrang of His. In a 

 series of figures the „Zwischenstrang" is easily shown to be just that 

 portion of the epiblast which fails to take any share in the formation 

 of the ganglion. — A Zwischenrinne does not exist either in the 

 head region of Elasmobranchs or of any other type. — On the con- 

 trary in head and trunk a Zwischenstrang can easily be found in 

 Sharks, Teleostei, Triton, Rana, Lacerta, Chick and Rabbit; and in 

 all cases it has the same relation to the formation of the ganglionic 

 Anlagen as I have described above. 



To the part of the ganglion formed just outside the neural plate 

 one may give the name of neural ganglion to distinguish it from 

 that part formed in connection with the lateral sense organs — the 

 latter may be known as lateral ganglion. While in the head 

 the ganglia arise from two sources, the spinal ganglia arise only from 

 one, and thus appear to be only neural ganglia in a strict mor- 

 phological sense. 



We left the ganglionic Anlagen lying between the unclosed lips 

 of the neural plate. Soon they begin to leave this position, and after 

 they have left it the neural plate can close into a tube. As long as 

 the ganglionic Anlagen are anywhere near the neural lips there is al- 

 ways a sharply defined boundary between the latter and the former. 

 The ganglionic Anlagen arc in no sense outgrowths of the neural plate. 



The neural ganglionic Anlage now grows towards the lateral epi- 

 l)last just above the gill cleft which is in course of formation. I am 

 here speaking of such a typical metamere as that containing the glosso- 



