48 KINGSLEY. [Vol. VII. 



region, and a posterior or thoracico-abdominal portion, the ce- 

 phalic being the smaller and more sharply differentiated from 

 the rest of the blastoderm. This occurs on the average about 

 fifteen days after impregnation. Twelve hours later a second 

 line occurs behind the first, cutting off from the thoracico- 

 abdominal region the first somite of the body ; and about twelve 

 hours later a third transverse line appears (Fig. 25), and now 

 there is a head region, two body somites, and an undifferentiated 

 caudal region. 



This figure (Fig. 25), taken from a blastoderm peeled from the 

 Qg^ and mounted in balsam, shows clearly that this appearance 

 of somites and lines of separation results from the fact that the 

 cells are abundant in certain regions and more scattered in 

 others ; in other words, from the outlining of the mesodermic 

 somites. This process continues until six segments behind 

 the head are formed, the sixth consisting of the united sixth 

 'thoracic' segment and the caudal plate.^ At first these seg- 

 ments are quite short and correspondingly broad (Fig. 26), but 

 later they increase rapidly in length. I may say in passing that 

 owing to the difficulties of observation it was not possible to be 

 certain of the limits and proportions in certain figures. Each 

 &^g had to be studied in strong sunlight, and the use of a 

 camera was impossible. Such was the case with Figs. 22, 

 23, 26. 



Mesoderm. 



The primitive cumulus is shown in section in Fig. 36. As 

 will be seen, the surface of the ^g^ is covered with a layer of 

 thin, flattened cells, while beneath are the entoderm cells. The 

 cumulus itself is thicker, partly owing to the fact that its com- 

 ponent cells are more columnar, and also to the fact that lower 

 layer cells have been formed. • The spot in the cumulus, which 

 in surface views (Fig. 21) looks like a pit, is seen in sections to 

 be produced by the greatly thickened centre of the cumulus. 



1 In a few instances I have seen reason to doubt this. In almost every instance 

 I have seen all six appendages arise at the same time, but in two or three cases, 

 {e.g. Fig. 28) but five appendages appear at first, the appendage i being noticeable 

 at a later date. This may indicate that the corresponding segment may be corre- 

 spondingly delayed; and that the above interpretation is not correct. On the 

 other hand, these instances may belong to some of the many anomalies, which 

 are found in examining a large series of Limulus embryos. 



