EARLY STAGES IN DEVELOPMENT OF BALANOGLOSSUS. 157 



EXPLANATION OP PLATES XIII, XIV, XV & XVI, 



Illustrating Mr. William Bateson's paper on " The Early Stages 

 in the Developmeut of Balanoglossus (Sp. incert.)." 



Comiilele List of Reference Letters. 



a. Anus. A. c. Arclieuteric cavily. b. c. 1, 2, and 3 ; first, second, and 

 third body cavities respectively. LI. Blastopore, bl. pi. Blastoporic pole. 

 c. Apical tuft of cilia, e.g. Groove separating the collar from the trunk. 

 cil. Transverse band of cilia, cl. Collar. D. The dorsal side. E. Epiblast. 

 for. 1, 2, 3 : foramina, by which first, second, and third body cavities respec- 

 tively communicale with the lumen of the gut. g. Groove separating proboscis 

 from the collar, g. s. Gill slit. //. Hypoblast. E'. Hypoblast beginning to 

 be differentiated to form the mesoblast of the prseoral lobe. H" . H'". Cells 

 still forming part of wall of archeuteron, but which are beginning to separate 

 off as the inner walls of the second and third body cavities respectively. 

 I. M'. Left posterior horn of anterior mesoblastic sac. (Vide note below.) 

 I. g. Longitudinal ventral groove in gut. m. Mouth. M'. M". 31'". Mesoblast 

 of first, second, and third regions, m.f. Muscle fibres forming in the meso- 

 blast of the prseoral lobe. ii. Commencing nervous system. ;z. c. Nerve 

 cord. 71. g. Neural groove, p. Proboscis, pt. Point at which anterior body 

 cavity closes, r. ]\J'. Right posterior horn of anterior body cavity. (Vide 

 note below.) s. c. Segmentation cavity, sh. Shell. 



I have to thank Mr. H. A. Chapman for assisting me with the figures of the 

 surface views. 



Fig. 31a was very kindly drawn for me by Mr. W. F. R. Weldon. 

 The outlines were drawn with Zeiss's camera lucida. 



Figs. 1 — 17. — Surface views of eggs and larvge of successive stages. 

 (Zeiss's Obj. A, oc. 2.) 



Fig. 1. — Ovum just laid, unfertilized. 

 Fig. 2. — Fertilized egg. 



[Note. — In interpreting those figures in which the right and left sid^s of 

 the animal are indicated, it must be remembered that as the figures are traced 

 as seen through the microscope, the position of the real right and left sides is 

 reversed.] 



