970 



Packard.] . '^'^ [Jan. 16, 



is a narrow cavity which sends a branch forward to the base of the abdo- 

 men, and a second obliquely downward and inward ; at 2 and 3 in fig. 2, 

 there are narrow cavities or splits (somatic cavities ?) wliich communicate 

 with a longitudinal internal opening.'which extends in a direction parallel 

 to the under (now outer) surface of the abdomen. In this respect the 

 embryo of Limulus is very different from that of the scorpion and spiders 

 (see especially Balfour's figs. 5, 6, PI. xix and fig. 15, PI. xx), where the 

 abdominal segments, witli their appendages and somatic cavhies are formed 

 contemporaneously with those of the cephalothorax. The innermost 

 mesodermic cells are now arranged in long cords, destined to form the 

 ventral adductor muscles of the abdomen. 



The mode of formation of the liead and its shape at this time presents 

 important differences from that of tracheate embryos. The procephalic 

 lobes are not developed ; the preoral portions of the head, i. e., that part 

 in front of the first pair of limbs is very small, short and narrow, merely 

 forming the end of the oral blastodermic disc, seen in my earlier published 

 figures. The structure of the preoral portion of the head {procephalum as we 

 may term it), is seen in longitudinal section in fig. 3, pc, to apparently 

 consist merely of an extension of the postoral part of the head ; with 

 apparently one or two splits in the mesoderm {ms^, ms^), the nature of 

 which I do not understand ; undoubtedly farther sections and comparisons 

 will throw light upon it. There is no involution of the epiblast, and the 

 section imsses one side of the mouth, a good section of whicli I have not 

 yet seen. 



Tlie first nervous ganglion is seen at fig. 5, to result (as also first shown 

 by Kingsley) in an ingrowth of the epiblast {inv. c) ; carrying into the 

 interior a mass of epiblastic nuclei, which envelop the myeloid substance 

 (my), which, as in older embryos, remains unstained by the carmine. 



The mesoblastic nuclei stop at a large cell (c), beyond which are long 

 incipient loose muscle-cells, with a few scattered nuclei. The procepha- 

 lum terminates abruptly, forming, as seen in our earlier figures already re- 

 ferred to, the end of the blastodermic disc. . 



The absence of the procephalic lobes in the embryo Limulus of this 

 stage seems to us to be a very significant fact, and to point to the early 

 divergence of the PalfBOcarida from the stem leading up to the Tracheata, 

 and especially the Arachnida. Metschnikofi's researches on Scorpio, with 

 those of Claparede, aud of Balfour on the spiders, and those of Sograff 

 on the myriopods, show that this is a fundamental and early attained fea- 

 ture in these types. Their absence in Limulus shows how little its embryo 

 has in common with tracheate embryos. At the same time the general 

 mode of formation of the blastodermic disc (ventral plate) of Limulus is 

 much like that of the spider, as seen in tlie mode of origin of the meso- 

 blastic segments, and the probable orgin of the hypoblastic cells. There 

 is a superficial resemblance between the embryo of Limulus and of the 

 spider, as may be seen by a comparison of our fig. 2, and Balfour's fig. 15. 

 Without much doubt the Tracheata and Pal^eocarida, as well as Crustacea 



