No. 2.] THE EMBRYOLOGY OF LIMULUS. 20I 



Patten describes an immigration of ectoderm cells from differ- 

 ent portions of the germinal area ; and lastly, the middle layer 

 receives further accessions from the marginal groove already 

 referred to above. The cells of this marginal area are de- 

 scribed as containing an extremely long, coiled, brilliantly 

 refractile filament ; and some of these cells become elongated 

 to form the dorsal muscles, the filament forming the longi- 

 tudinal striation. 



I have not seen, either in surface views or in sections, the 

 slit-like invaginations described by Professor Patten ; I have 

 seen no additions to the mesoderm from various points of the 

 blastoderm, outside of the limits of the primitive streak ; I 

 have not seen any cells, much less great masses of them, 

 separate from the primitive streak and wander into and become 

 scattered through the yolk ; and I have yet to see the peculiar 

 origin of the dorsal muscle-cells which he describes. He 

 figures (p. 374 Fig. i8 D, pstr^ a cylindrical rod of invaginative 

 tissue, an appearance lacking in my sections, and refers to 

 masses of entoderm cells at the inner end of the oesophagus 

 which are quickly absorbed and which I am confident do not 

 exist. 



Kishinouye ('92) has the mesoderm in Lhfuiliis longispina 

 arising from three sources : — from the cells forming the lower 

 part of the blastoderm thickening ; from the primitive streak ; 

 and from the yolk cells. The first portion forms the mesoderm 

 of the cephalothorax ; the second the mesoderm of the abdo- 

 men ; the last probably the blood corpuscles. I must be per- 

 mitted to express my scepticism upon this account of the 

 origin and fate of the different portions of the middle layer. 

 In the subsequent history however we are in close accord and 

 only the points of difference need be noted. The order of the 

 appearance of the mesoderm somites is the same and in them 

 the coelom appears as schizocoelia. According to Kishinouye 

 no coelomic cavities appear in somites II, III, or IV, and in 

 the others they do not extend into the appendages. In both 

 points the Atlantic Limulus differs ; cavities occurring in 

 every postoral somite (Fig. 46) and at first extending into the 

 limbs. In both species the coelom (or at least a part of each 



