No. I.] THE EMBRYOLOGY OF LIMULUS. ^j 



The transition from the Japanese Agelena and Scorpio to a 

 true delamination is greater than that already traced ; and as yet, 

 so far as the literature at hand enables me to decide, it cannot 

 be traced without going outside the limited group of Arachnids. 

 Still the successive stages are readily imagined. 



In the ectolecithal egg the blastoderm arises by migration of 

 the primitively central cells to the periphery, and in many 

 forms every nucleus goes through this migration, leaving the 

 yolk at one period entirely free from cells. In others only a 

 portion of the cells reach the surface, the others remaining 

 behind in the yolk. Concerning the fate of these latter, opinions 

 differ. In some forms they are described as playing no part in 

 the building up of the embryo, but rather acting as * vitellophags,' 

 the sole function of which is to gradually metabolize the deuto- 

 plasm, after which they disappear. On the other hand, in- 

 stances are not wanting in which these yolk cells are to be 

 regarded as true entoderm cells, from which later the epithelium 

 of the mid-gut is to be built up. This is the case with Limulus, 

 as I shall detail later, and apparently also in many Araneina and 

 Hexapods. 



With such conditions as are afforded by Crangon, Theridion, 

 etc., it can readily be seen that any acceleration of develop- 

 ment which would prevent certain of the central blastomeres 

 from migrating to the surface, only to be immediately returned 

 as entoderm, would be a distinct gain ; and this, in my opinion, 

 is the way the peculiar conditions in many Hexapods have been 

 brought about. At least, this view has the merit of rendering 

 intelligible many features of Arthropod ontogeny which other- 

 wise are not readily understood. 



A farther step in the same direction is afforded by Limulus, 

 where a farther economy is seen in the cutting off of the periph- 

 eral from the deeper ends of the cells, thus at once differen- 

 tiating an outer ecto-mesodermal layer from an inner entoderm 

 rich in food yolk. The final stage, as we know it, is seen in 

 Tanystylum and Phoxichilidium as described by Morgan ('90). 

 Here the ^^■g is much reduced in size, the blastomeres are 

 fewer, and each cell is at once (apparently) differentiated into 

 entodermal and ecto-mesodermal portions, the result being a 

 condition which closely simulates the multipolar delamination 

 found in Geryonia, made classic by the researches of Fol and 



