2g Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 



Some Variations in External Structures. 



Before the individual variations are considered in detail it should be stated that a given nest contains 

 but few eggs that are in precisely the same stage of development. The difFerences are inost obvious in the 

 early stages up to the closure of the neural fold. PVom the closure of the neural folds to the 39 mm larva 

 the variations are not so pronounced, yet there are innumerable minor variations. 



In the following descriptions the principal variations observed in the external features are first 

 recorded, then those observed in the sections. 



Cleavage. 



In the cleavage stages, from the second on to late cleavage, there is so much Variation in the 

 Position, extent and rate of progress of the various grooves that it is impossible to record them. Some of 

 the variations have been described elsewhere (1904 b) by the senior author. 



Gastrulation. 



Some Variation is found in the position of the first line of invagination which forms the dorsal lip 

 of the blastopore. Its first appearance may be along a line equidistant from the equator and the vegetative 

 pole or it may form nearer the equator and again sometimes nearer the vegetative pole. The first line of 

 invagination may be nearly straight and again it may be decidedly crescentic. The maximal diameter of 

 the yolk plug may equal one half the diameter of the egg. The closure of the blastopore usually occurs 

 in about six days but it may close in five days. 



Closure of neural folds. 



Variations in the closure of the neural fold are frequently observed. The coalescence of the lateral 

 portions of the fold usually begins at the posterior end. Sometimes they first coalesce along the middle 

 portion of the embryo. Usually the transverse portion of the fold is continuous ; at other times it shows a 

 deep transverse groove which separates it into right and left halves. Cf. Figs. 18, 19, 20, 21. Wide variations 

 exist in depth, width and extent of the neural groove. 



Appearance of optic vesicles. 



The optic vesicles are usually present before the complete closure of the neural fold. Sometimes 

 tliey are present as disc-like depressions in the neural plate before the lateral portions of the fold have 

 begun to coalesce; again, but rarely, they are not visible until the folds have closed. 



Neuromeres (?). 



There are frequently observed in the cephalic region well marked serial elevations and depressions 

 along the inner margins of the lateral folds and across the neural plate. In other embryos there is not the 

 slightest trace of either. 



