No. 2.] THE EMBRYOLOGY OF LIMULUS. 22 1 



Schulgin ('86) and Laurie {'90) show that in the early stages the 

 entoderm is a distinct differentiation from the germinal area ; 

 it remains for a long time as a solid mass at the posterior end 

 of the embryo and only later spreads itself out as a definite 

 layer enclosing the yolk and forming the epithelium of the 

 mid2:ut and its diverticula. Of the details of the formation of 

 the liver-lobes but little is said, but apparently it occurs through 

 the ingrowth of the mesodermal septa as in Araneina and in 

 Limulus. The different type of formation of the entoderm in 

 the scorpions and Limulus is possibly the strongest objection 

 which can be raised to the close association of the two forms, 

 for while the two types can be reconciled, it implies an ex- 

 tremely long separation of the forms. Still it must be borne 

 in mind that if this be advanced as an objection it must be 

 equally valid in proving that the Araneina and the scorpions 

 are but remotely connected, a thesis which I hardly care to 

 defend, while on the other hand it would show closer relation- 

 ship between Limulus and the spiders than between the former 

 and the scorpions, a view which is negatived by numerous 

 other facts of structure and ontogeny. 



Limulus also agrees with the Arachnida in the early appear- 

 ance and elongation of the stomodaeum and the shortness and 

 lateness of appearance of the proctodaeum. As yet, no obser- 

 vations are recorded as to the manner of invagination of the 

 stomodseum, but as figured by Morin (in Korschelt and Heider 

 '92) and Locy ('86) it bears close resemblance to that of Limulus 

 in its bending and in the early differentiation of a terminal 

 pouch clearly comparable to the proventriculus of Limulus. In 

 the only sagittal section given by Laurie of the later stage of 

 the stomodaeum in the scorpion ('90, PI. XVII, Fig. 48) the 

 proventricular enlargement is not very apparent but the buccal 

 cavity is well marked. Concerning the proctodeal region the 

 comparisons reveal little of importance. In the spiders we 

 have in this region the formation of the stercoral pocket ^ 

 which is without parallel in Limulus {vide, however, siipra p. 



219 and Fig. ^Z). 



t 



1 Kishinouye's observation ('90, p. (&) that the stercoral pocket is developed 

 from the unpaired coelom of the caudal region certainly needs confirmation. 



