ON THE DEVRLOPMENT OF SAGTTTA. 373 



tions took place. The further development, leading to the 

 adult condition, has therefore to be studied in specimens 

 taken in the tow-net, and the consideration of it will be post- 

 poned until after the sections of the early young have been 

 discussed. 



VI. Further Stody op Larval Development by means 

 or Sections. 



The general course of the development after hatching up 

 to the end of the second week has been described in the last 

 section, and therefore only those points will be dealt with 

 here which are better seen in section than in the living or 

 mounted animal. It will be most convenient to consider first 

 the sections of the head at different stages, and afterwards 

 those of the trunk and tail. 



A transverse section through the month region of a larva 

 killed a few hours after hatching differs little from that 

 through the embryonic head ; it is rather smaller, owing to 

 the lengthening of the body at the expense of its Avidth, and 

 it is also becoming more obvious that the ectoderm at the 

 sides of the head is becoming thickened and consists of more 

 than one layer of cells (hd.). 



On the second day (fig. 22) little change has taken place ; 

 the nuclei on the ventro-lateral parts of the ectoderm are 

 more numerous, and at the sides of the head that layer shows 

 signs of splitting, but otherwise the structure is closely 

 similar to that of the first-day larva. By the third day, 

 however, development has proceeded considerably ; the 

 tissues are beginning to take on their permanent form, and 

 in consequence the definite structure of the head becomes 

 marked out. 



Beginning with the ectoderm, it is seen in fig. 23 that the 

 hood has now been formed by a splitting off of the two outer 

 cell layers at the sides of the head, but on the dorsal surface 

 they remain continuous with the general ectoderm, and so 



