EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



inside out and upside down, and reverses its axes of growth and of specialization. 

 Gaskell entered the field of embryology as a novice, and at once became 

 hopelessly confused by the conflicting usage of the terms "dorsal" and "ventral," 

 and he still remains so, because he did not establish a fixed basis for orientation. 

 The result is familiar enough. In spite of the testimony of his own senses, all 

 his rivers persist in flowing up hill, and the north pole of his compass points due 

 south. 



Gaskell at least makes a valiant fight to save the pieces of the invertebrate 

 nervous system, even if he does annihilate the rest of the animal in the attempt. 

 Prof. C. J. Herrick's effort is not as praiseworthy, since he discards altogether a 

 "perfectly good" nervous system, and substitutes for it a new one created out of 

 empty space. 



III. EARLY STAGES OF LIMULUS. 



The development of Limulus will serve as a convenient basis for a comparative 

 study of the embryology of vertebrates and arachnids. I have given little atten- 

 tion to the maturation, fertilization, and cleavage, devoting most of my time to 

 the general form of the body at successive stages, and to the method of growth of 

 the various organs. 



Observations were made on the living eggs during the cleavage and gastrula 

 stages. But most of the drawings, up to the appearance of the appendages, 

 were made from eggs hardened in picro-nitric, or Perenys fluid, and viewed as 

 opaque objects, after removing the membranes. The older stages were drawn 

 from embryos that had been stained in various ways, and cleared in cedar oil or 

 balsam. 



The earlier stages w r ere most conveniently obtained by artificial fertilization; 

 the later stages w r ere obtained from the nests in the sandy beaches of Woods Hole, 

 where, in 1893-94, most of the embryological work on Limulus was done. 



In artificial fertilization, the female is opened and the eggs poured into a 

 shallow glass dish. By carefully tilting from side to side, the ripe eggs may be 

 made to adhere to the bottom and sides in a compact single layer. After a few 

 minutes they may be rinsed off in fresh sea water, removing the- clotted blood and 

 immature eggs, and then fertilized. 



The eggs in some crabs are a dull slate color, in others pink or buff, or of 

 various intermediate shades. When first taken from the body, thev are shriveled 



j ' J 



and distorted, but after a short time in sea water, they become plump and round. 

 During the first thirty-six hours after fertilization, their form and appearance 

 change rapidly. At eighteen hours, most of them appeared to segment into two 

 unequal blastomeres. (Fig. 122, a.) About two hours later, they again assumed 

 a regular outline, some of them meantime showing on the upper surface a radiate 

 appearance which soon disappeared. (Fig. ] 22, b.) 



