356 



50 Faden Tiefe von 2-1— 2ü"0. Oberflächentemperatur auf 15° C, bei 

 100 Faden auf 11—12° C, bei 300—400 Faden auf 6° C. Ein Verhält- 

 nis, das dazu beiträgt, schon der Fauna geringer Tiefen den Character 

 der gemäßigten Zone zu geben und die Abyssalfauna bis auf 360 Fa- 

 den aufsteiiien zu lassen. 



2. Note on the function of the »test-cells» in Ascidian Ova. 



by Prof. J. Play fair McMurrich, Guelph Canada. 



Numerous, and to a certain extent contradictory, observations have 

 at various times been made on the so-called »test- cells« so characte- 

 ristic of the ova of Tunicates. At one time it was thought (and hence 

 the designation of these structures) that they eventually formed the 

 test enveloping the Ascidian, but that theory has been most effectually 

 overthrown and since, as far as I can ascertain, no explanation of their 

 appearance and function has been promulgated. 



In the last number of the »Studies from the Biological Laboratory« 

 (Vol.11, No. 2) published by the Johns Hopkins University; Baltimore, 

 U.S., I had the honour of recording some observations on the forma- 

 tion of the »test- cells«, made during the past summer at the Marine 

 Zoological Station in connection with that University. Therein 1 show- 

 ed that these structures were produced by a contraction of the yolk 

 of the ovum consequent on the action of various stimuli, being formed 

 more or less distinctly according as the stimulus was capable of causing 

 a greater or less contraction of the egg contents. 



I now desire to enunciate a theory as to their function. The la- 

 test theories on the subject of parthenogenesis and of the nature of 

 polar-globules are based on the assumption of the bisexual nature of 

 the ovum, on account of which it is possible, and there is even a ten- 

 dency, for the yolk to divide spontaneously. In most cases this is dis- 

 advantageous, and the formation of »test -cells« is a means of guarding 

 against the misfortune. On the exposure of the ova to sea-water or 

 other abnormal condition a contraction of the yolk is brought about, 

 and thereby a tension upon the nucleus, which under the strain to 

 which it is subjected would divide, and so start the process of segmen- 

 tation, were that strain not removed from it by the extrusion of the 

 »test -cells«, whereby it is preserved intact until the proper stimulus in 

 the shape of a spermatozoon excites it to a healthy and normal division. 



This theory I would also suggest as an explanation of the »excret- 

 körper« described by Her twig and O el la cher as appearing in the ova 

 of Amphibia and Fish respectively, and also of the fatty globules de- 



