EARLY STAGES IN DEVELOPMENT OF BALANOGLOSSUS. 233 

 blance are more than superficial, especially when it is re- 

 membered that the adults of both animals possess essentially 

 similar branchial structures which, beyond the Chordata, are 

 otherwise without parallel in the animal kingdom. There are, 

 of course, many and great difficulties which preclude any 

 assumption of relationship between them, notably the absence 

 of any regular notochord in Balanoglossus. 



On this occasion it is not profitable to discuss these ques- 

 tions at greater length. When, however, the later develop- 

 ment of this form has been described, it may perhaps be 

 possible to arrive at more definite conclusions. 



In closing this paper I have to thank Mr. Sedgwick and 

 Mr. Weldon for rendering me much valuable assistance and 



advice in connection with it. 



EXPLANATION OF PLATES XVIII, XIX, XX, & XXI, 



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

 in the Development of Balanoglossus (Sp. in cert.)." 



Complete List of Reference Letters. 



a Alius. A. c. Arcbenteric cavity, b. c. I, 2, and 3 : first, second, and 

 third body cavities respectively. bl. Blastopore, bl.pl. Blastoponc pole. 

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

 cil Transverse band of cilia, el. Collar. D. The dorsal side. K Lpiblast. 

 for \ 2 2>: foramina, by which first, second, and third body cavities respec- 

 tively communicate with the lumen of the gut. g. Groove separatins proboscis 

 from the collar. ^. .. Gill slit. //.Hypoblast. ^'Hypoblast beginning to 

 be differentiated to form the mesoblast of the preeoral lobe. E .U . bells 

 still forming part of wall of archenteron, 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.) 

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

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

 blast of the prreoral lobe. n. Commencing nervous system. «. c. JNerve 



