The shield is partly a portion of the skeleton, and partly an ossification of the skin or of the walls of the body.
The second volume treats entirely of the embryology of the turtle, and is illustrated with thirty-four plates. The egg originates from between the cells of the stroma, and is in itself the animal in the first stages of development. The eggs are laid once a year, and grow a long time before they are fecundated. From the first copulation to the laying, four years elapse, during which time eight copulations take place. The segmentation of the yolk takes place during the passage of the egg through the oviduct. From the segmentation of the yolk to the period of hatching, the egg passes through thirty-one stages of development.
The third volume of the "Contributions" appeared in 1860, and was devoted to the class of Acalephæ, the author treating specially of the order Ctenophoræ'. The Ctenophoræ Agassiz divided into three suborders: the Eurystomæ', embracing three families, the Saccatæ, with three families, and the Lobatæ, with five families. The fourth volume of the "Contributions," in 1862, concluded the Acalephæ, treating of the Discophoræ, Hydroidæ, and the Homologies of the Radiata.
Of the more recent labors of Agassiz in connection with the Museum of Comparative Zoology at Cambridge—of which he was the director, and to which, in later years, he devoted his whole attention—it is not necessary to speak. With his journey to Brazil in 1865, and his later expedition from Boston to San Francisco on the United States Coast Survey steamer Hassler in 1872, the public is already sufficiently familiar. His last days were devoted to the cause of education, in the establishment of a school of natural history at Penikese Island.
Of the man himself but a word is necessary. As a naturalist, Prof. Agassiz was unwearied in his devotion to his favorite pursuits. He worked early and late, often denying to himself the most necessary rest and recreation; and his remarkably strong constitution sustained him under a strain that would quickly have proved fatal to a man of less vigor. His mind was preëminently great; gifted with a wonderfully retentive memory, he combined with it a power of generalization and quick perception that places him next to Cuvier, whose disciple he was, and whom he seemed to imitate. In his methods of investigation he was perfectly honest, and, though many might differ from him in his conclusions, none could deny the absolute integrity of his convictions. In his intercourse with his fellow-men he was extremely affable and genial, and especially so toward the young. With inexperience he was most patient and painstaking, never wearying in his efforts to aid. Tolerant of ignorance where associated with modesty, he had but little patience with arrogance and ignorance combined. His students will all bear witness to the unvarying cheerfulness and ready sympathy in him they had learned to look up to as their master.