104 INVERTEBRATA CHAP. 



The preserving fluids employed with most success were a mixture 

 of 3 parts corrosive sublimate and 1 part acetic acid dissolved in 95 per 

 cent alcohol ; and Gilson's fluid a mixture of corrosive sublimate and 

 nitric acid. Other fluids were tried but did not give good results. 

 The preserved eggs were stained in Grenadier's haematoxylin, and 

 cleared in xylol and mounted in Canada balsam. The operation of the 

 clearing agent was facilitated by piercing the capsule with needles. 



The embryo develops within the capsule until the sixth day, 

 when it escapes as a free-swimming larva of a peculiar type known 

 as Mliller's larva. This larva swims about at the surface for some 

 time, but eventually sinks to the bottom and then takes on the form 

 and habits of a Polyclade. 



In the development of Planocera we meet for the first time with 

 what is termed " spiral cleavage " of the egg. This type of segmen- 

 tation of the egg into blastomeres is very widely distributed. It is 

 found in all Polyclada which have been studied, it is universal 

 amongst Annelida, and it is found in all Mollusca except Cephalo- 

 poda. In the Neuiertine worms it also appears, though in a very 

 primitive form. Developments of this type have been studied in 

 great detail by zoologists of the American school, and they have 

 invented a nomenclature which is applicable to all such develop- 

 ments ; we shall endeavour to make this clear, and shall adopt it in 

 the description which we give. The object aimed at in these studies 

 is to trace back definite organs of the embryo or larva to individual 

 blastorneres of the egg in its early stages of cleavage. This is termed 

 tracing the Cell-lineage of the organs. 



The general features of "spiral cleavage" are these. The egg 

 divides as usual into two and then into four cells, by two cleavage 

 planes at right angles to one another. These four cells then divide 

 into eight, which lie in two tiers of four, one above the other. So 

 far there is nothing peculiar about the type. But the four cells nearest 

 the animal pole of the egg are usually much smaller than the 

 others, and are termed micromeres, whilst their larger sisters are 

 termed macromeres. Further, the spindles by which the nuclei of 

 the micromeres are separated from those of the macromeres are not 

 vertical but oblique, with the result that the micromeres are situated 

 opposite the furrows between the macromeres, and, so to speak, 

 alternate with them. 



In Planocera, and in the great majority of cases of spiral cleavage, in 

 the transition from the 4- to the 8 -cell stage, each spindle is so 

 inclined that the micrornere lies at the upper right-hand corner of the 

 macromere from which it has separated (compare 1A and la in Fig. 

 80) ; that is, the upper right-hand corner when viewing the egg from 

 above the animal pole. Such a direction of cleavage is termed 

 dexiotropic. In only a few cases, in which reversed cleavage, as it 

 is called, occurs, the spindles preparatory to the formation of the 

 8-cell stage are so directed that the micromere lies at the left-hand 

 upper corner of the macromere. Such cleavages are termed laeotropic. 



