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Opilionid defense glands consist of 0.5 × 0.9-mm sacs attached to the underside of low tubercles located on the dorsal side of the cephalothorax, posterior to the first pair of legs. Each gland opens via an elongated slit, located in the posterior floor of a crater that is situated at the summit of the tubercle. The center of the sac, called the reservoir, is lined by a cuticle consisting of epicuticle and endocuticle which is continuous through the slit with the exoskeleton. The layers of cuticle vary in thickness with different locations in the gland. A hemocoelomic (basement) membrane, 0.5-1, μ thick, forms the boundary between glandular cells and hemocoel. The gland has a nonsecretory portion consisting only of cuticle-supporting cells and a secretory portion consisting of secretory and cuticle-supporting cells. The cuticle lining the reservoir in the secretory area is broached by many cuticle-lined ductules, each of which drains an isolated intercellular space called the intercalated cistern. This in turn drains microvilli-lined canaliculi located between and extending into secretory cells. The cisterns are devoid of microvilli. Secretory cell cytoplasm contains a Golgi apparatus, many free ribosomes, rough endoplasmic reticulum (RER), two types of granules (speckled and dense), and mitochondria. Speckled granules are partially filled with fairly large particles and are found in association with the Golgi apparatus. They also surround canaliculi into which they empty. Dense granules are packed with very small particles, have a gray homogeneous appearance, and are scattered throughout the cytoplasm. Mitochondria containing matrix granules tend to scatter throughout the cytoplasm but are concentrated around canaliculi.

The germinal crescent in the chirk embryo is characterized by small, PAS-positive, nonglycogen granules from 1.5 to 5 4 in diameter. The primordial germ rells (PGCs) were found to originate in and separate from the germinal crescent endoderm through stage 7 (2 somites). Shortly after separation most of the granules in the PGCs lost their organization and the PAS-positive material was distributed irregularly throughout the cytoplasm. A few of these granules remained within the cells indefinitely. Glycogen of an agranular nature which had shifted to one pole of the cell was observed a t stage four. Granular glycogen which was distrihuted throughout the rytoplasm was not observed prior to stage 7 or 8.Cell counts on individual embryos showed noticeable variations as to the number of germ cells between embryos of the same stage. For example, in stage 4 embryos the minimum number of cells rounted, including attached and free, was 78 and the maximum 169, while in stage 9 the minimum was 83 and the maximum 469 cells. After separation the germ cells were observed almost anywhere between the ectoderm and the endoderm although the majority remained in the area whew they originated.Swift ('14) concluded that the primordial germ cells (PGCs) were derived from the germ wall endoderm, a t its junction with the area pellucida, anterior and antero-lateral to the forming embryo. Because of its shape, this area became known as the germinal crescent and the germ cells were found t o originate in i t during the primitive streak and up through the three somite stage. These cells then entered the forming blood vessels and with the establishment of circulation were carried to the gonadal site. These observations were confirmed by Goldsmith ('28) in the chick and by Blocker ('33) in the English sparrow.Further proof of the area of origin of the PGCs in the chick was provided by the experiments of Goldsmith ('35) and Simon ('57a) in which sterile embryos resulted from removal of the germinal crescent.The in vitro experiments of Simon ('57b) demonstrated the role that the circulation plays in the migration of the germ cells. This investigator placed a blastodisc, from which the germinal crescent had been re-AM. J. ANAT., 185: 87-112.

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