ILLUSTRATIONS xxxi 



FIG. PAI7E 



415. Early stages in the development of Balanoglossus davigerus . . 571 



416. Later stages in the development of Balanoglossus davigerus . . 572 



417. Still later stages in the development of Balanoglossus davigerus . . 573 



418. Surface view of the young Tornaria larva of Balanoglossus davigerus, four 



days old ......... 574 



419. Illustrating the structure of the apical plate and eyes of a full-grown New 



England Tornaria larva . . . . . . 575 



420. Illustrating the origin of the middle and posterior coelornic vesicles in the 



New England Tornaria ....... 576 



421. Full-grown New England Tornaria, seen from the left side . . 577 



422. Illustrating the metamorphosis of the Bahamas Tornaria . . . 578 



423. Illustrating the development of the dorsal nervous system in the meta- 



morphosing Bahamas Tornaria ...... 579 



424. Sagittal section through the New England Toruaria immediately after 



the metamorphosis into the Balanoglossid worm .... 580 



425. Portions of transverse sections through the trunk region of three young 



Balanoglossid worms of different ages in order to illustrate the develop- 

 ment of the genital organs in the Bahamas species . . . 581 



426. The larva of Dolichoglossus pusillus in the act of escaping from the egg- 



membrane ......... 583 



427. Longitudinal frontal sections through two embryos of Dolichoglossus 



pusillus in order to illustrate the development of the body-cavities . 584 



428. Stages in the segmentation of the egg of Amphioxus lanceolatus . . 588 



429. Stages in the gastrulation of the egg of Amphioxus lanceolatus. All the 



figures represent sagittal sections ...... 590 



430. Diagrams showing the parts played by ectoderm and endoderm in the 



process of gastrulation in Amphioxus lanceolatus .... 592 



431. Transverse sections through an embryo of Amphioxus lanceolatus just 



after the completion of gastrulation in order to illustrate the develop- 

 ment of nerve-cord, notochord, and coelom .... 593 



432. Transverse sections through an older embryo of Amphioxus lanceolatus 



than that represented in the previous figure to show the further develop- 

 ment of nerve-cord, notochord, and coelom .... 594 



433. Illustrating the formation and development of the head-cavities of 



Amphioxus lanceolatus ....... 595 



434. Diagram of longitudinal section of Amphioxus lanceolatus parallel to the 



sagittal plane but lying to one side of the middle line in order to show 



the mutual relationships of head, collar, and trunk-cavities . . 596 



435. Illustrating the mutual relationships of collar-cavities and splanchnocoeles 



in Amphioxus lanceolatus ... ... 597 



436. Two young larvae of Amphioxus lanceolatus, seen from the left side . 598 



437. Larva of Amphioxus lanceolatus with four primary gill-slits, seen from the 



left side ......... 599 



438. Diagrammatic transverse sections through the hinder region of two young 



specimens of Amphioxus lanceolatus in order to show the origin of 

 sclerotomes ......... 600 



439. Illustrating the development of the excretory organs of Amphioxus 



lanceolatus ......... 601 



440. Illustrating the development of the genital organs of Amphioxus lanceolatus 602 



441. Illustrating the metamorphosis of the larva of Amphioxus lanceolatus . 603 



442. The egg of Cynthia partita before and during fertilization . . . 610 



443. Stages in the segmentation of the egg of Cynthia partita . . . 612 



444. Illustrating the gastrulation of the egg of Cynthia partita . . . 615 



