THE FORMATION OF THE GERM-LAYERS. 



119 



the Decapoda after the completion of the Nauplius stage are equivalent to' 

 larval integuments (Van Beneden, No. 79). In the Decapoda, a second larval 

 integument is often developed in later stages ; this surrounds the hatching 

 Zoaea, and was assumed by Conn to be the cuticle of the Protozoaea stage. 

 In Anchorella and Lernacopoda (Van Beneden, No. 17), the embryo moults 

 three times during the course of its development: (1) When the blastodermic 

 cuticle is formed, (2) when the Nauplius cuticle develops, and (3) during the 

 transition to the Cyclo^^s stage. >^Dohrn has bestowed special attention on 

 the subject of larval integuments in his various works {cf. his treatise on the 

 larval integument of the Cumacea, of Tannis, and on the Nauplius stage in 

 the egg of Daphnia longispina). On account of tl o great variety prevailing 

 among the Crustacea, and the uncertainty in the identification of the cuticles 

 in the different cases, we should be overstepping the limits of this work were 

 we to enter upon all the cases illustrating tliis point. 



The peculiar processes which were observed in the winter eggs of many 

 Cladocera by Weismann and Ischikawa (No. 16), and described as para- 

 copulation, synchronise with the phenomena of cleavage. Here, after the 

 ejection of the polar bodies and subsequent fertilisation, there is found a body 

 resembling a nucleus surrounded with an accumulation of protoplasm ; this 

 is called the eopulation-ccll. During the first division of the cleavage-nucleus, 

 by which the purely superficial segmentation of the^pe (III.) is introduced, 

 the copulation-cell remains apparently jjassive near the vegetative pole of the 

 egg ; it, however, soon approaches one of the nuclei which result from the 

 division and becomes completely fused with it. The further fate of the cleavage- 

 nucleus thus entering into paracopulatiou has not been traced. The view that 

 it is destined to yield the genital rudiments is a mere assumption. The copu- 

 lation-cell appears first at the time of the formation of the egg. "When the 

 latter is maturing in the ovary, chromatin particles are ejected from the germinal 

 vesicle ; these unite to form the nucleus of the copulation-cell, which, at a later 

 stage, becomes surrounded by a mass of protoplasm, probably arising from the 

 cell-body. So far no hypothesis has been formed as to the significance of 

 the processes of paracopulatiou. The origin of the copulation-cell recalls the 

 ejection of chromatin particles by the germinal vesicle observed by Stuhlmann 

 and Blochmann in the eggs of insects. Similar processes have also been noted 

 in the maturing eggs of Myriopoda (Balbiani) and Arancae (Leydig) and 

 in other groups of animals. 



3. The Formation of the Germ-layers. 

 A. Copepoda. 



Among all the Crustacea, so far as our present knowledge of their 

 ontogeny enables us to judge, the Copepoda most closely resemble 

 the Annelida in their development. In them we have an invagi- 

 nation -gastrula and the formation of the mesoderm through the 

 separation of two primitive mesoderm-cells. The formation of the 

 germinal layers in the Copepoda was made known by the researches 

 of Grobben (No. 21), Hoek (No. 22), and Urbanowicz (Nos. 23 

 and 24). We have utilised the minute observations of the first 

 of these writers in the account of Cetochilus here given. 



