492 SUMMARY OF CURRENT RESEARCHES RELATING TO 



eventually formed the test enveloping the Ascidian. This view was 

 shown to be erroneous, and Professor J. P. McMurrich now enunciates 

 a new theory as to their function.* 



The latest 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 tendency, for a yolk to divide spontaneously. In most cases 

 this is disadvantageous, 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 pro- 

 cess of segmentation, 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 the author would also suggest as an explanation of the 

 Excretkbrper described by Hertwig and Oellacher as aj^pearing in 

 the ova of Amphibia and fish respectively, and also of the fatty 

 globules described by the late Sir Wyville Thomson as occurring in the 

 eggs of Comatula, to which structures test-cells bear no little 

 resemblance. 



Embryology of the Bryozoa.f — J. Barrois finds that the larva of 

 a Bryozoon consists essentially of five principal parts; an aboral 

 surface, the peripheral part of the oral surface with the corona which 

 is only the edge of it, the incubating pouch with the central part of 

 the oral surface which is destined to form the intra-tentacular space, 

 the intestine, and lastly, the rudiment of the polypite which already 

 exists in the larva, where it forms a special organ, and takes more or 

 less a part in the formation of the polypite. 



In the Entoprocta these parts have most nearly the arrangement 

 which is found in the adult ; the aboral surface forms the integument 

 of the larva, and the oral is retractile and can be withdrawn into the 

 vestibule ; the only change necessary to convert the larva into the 

 adult is a rotation of the incubatory pouch and the intestine so as to 

 bring them into relation with the rudiment of the polypite. In the 

 Chilostomata there is developed, by the aboral growth of the corona, 

 a pallial cavity ; — as the oral surface has here lost its retractile power, 

 there must be a change in the position of the mantle before the larva 

 can pass into the adult condition ; here, therefore, there is a more 

 marked metamorphosis. In the Ctenostomata the pallial cavity is 

 enormous, and the cells of the corona are of very large size ; in the 

 Cyclostomata there is no corona, but the oral surface continues to 

 grow towards the aboral pole ; and here, therefore, we have, in its 

 most marked condition, the process which has become more and more 



* In a previous paper he showed that the test-cells were produced by a con- 

 traction 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. 



t Journ. Anat. et Physiol. (Robin) xviii. (1882) pp. 124-G1 (1 pi.). 



