of a Species of Chironomus. Ill 



On examining the ovum at this period from the ventral sur- 

 face, it is easy to perceive that the nutritive vitellus has 

 acquired a form different from the original one ; it has become 

 attenuated at each end and gibbous in the middle, and at the 

 same time much lighter in its posterior half, where the caudal 

 pad is situated, which is caused by its less thickness, because 

 here, as has been said, the pad of the germinal streak has 

 consumed it. Then, whilst the caudal pad is constantly en- 

 larging, the opposite cephalic ridge is formed. 



As the caudal pad becomes elongated, it ascends nearly to 

 the middle of the ovum, i. e. approaches the cephalic end of 

 the ovum. At this time there rises upon its dorsal surface a 

 transverse elevation, the margin of which is turned towards 

 the inferior pole of the ovum. This elevation, growing, be- 

 comes converted into a fold which covers half the caudal pad. 

 This is the so-called caudal fold. 



As regards the whole blastodermic mass, this thickens in 

 the ventral side of the head, and becomes attenuated in the 

 opposite or dorsal side. At the time when the dorsal blasto- 

 derm has attained its minimum thickness, a curved dark 

 streak is to be seen, from the dorsal sm'face of the ovum, upon 

 the nutritive vitelline mass ; its dark colour is due to the more 

 considerable vitelline mass here placed, or to the blastodermic 

 layer being most attenuated here. This streak very soon 

 acquires more distinct limits and a still darker colour. At 

 this time occurs the so-called bursting of the blastoderm, upon 

 which Weismann has established that type of development 

 which he designates by the name of " regmagene." 



During this bursting, the caudal fold has already grown far 

 downward, and at the moment of bm'sting it embraces the 

 caudal pad in its whole thickness. 



The whole embryo moves 180° upon its longitudinal axis ; 

 so that its belly comes to lie in the flat side of the ovum, and 

 its back in the convex one. 



Somewhat later we observe a diminution of thickness in the 

 nutritive vitelline mass lying in the cephalic extremity, i. e. a 

 thickening of the cephalic portion of the germinal streak, the 

 formation of the cephalic hood. Here a fold is then formed, 

 the margin of which is directed towards the superior pole of 

 the ovum. It grows much more rapidly than the caudal fold, 

 so that the margins of these two folds reach the middle of the 

 embryo at the same time. As their margins grow together, 

 they now form only one fold, which covers the whole dorsal 

 surface of the embiyo. With the growth of this fold {i. e. the 

 embryonal envelope) the place of origin of the caudal fold has 

 moved far up, and it soon occupies the free space formed by 



